tEMENTS  OF  GEOLOGY 


NORTON 


GIFT  OF 
Agriculture  education 


THE 


ELEMENTS  OF  GEOLOGY 


BY 
WILLIAM   HARMON  NORTON 

PBOFESSOK  OF  GEOLOGY  is  COBNELL  COLLEGE 


GINN  AND  COMPANY 

BOSTON    •    NEW   YORK     •    CHICAGO     •    LONDON 
ATLANTA    •    DALLAS    •    COLUMBUS     •    SAN    FRANCISCO 


COPYRIGHT,  1905,  1921,  BY 
WILLIAM   HARMON  NORTON 


ALL   RIGHTS  RESERVED 

--r 

321.5 

AGRIC.  DEFT. 


fltftenatum 


(iINN  AND  COMPANY  •  PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


PKEFACE 

Geology  is  a  science  of  such  rapid  growth  that  no  apology  is 
expected  when  from  time  to  time  a  new  text-book  is  added  to 
those  already  in  the  field.  The  present  work,  however,  is  the 
outcome  of  the  need  of  a  text-book  of  very  simple  outline,  in 
which  causes  and  their  consequences  should  be  knit  together  as 
closely  as  possible,  —  a  need  long  felt  by  the  author  hi  his  teach- 
ing, and  perhaps  by  other  teachers  also.  The  author  has  ven- 
tured, therefore,  to  depart  from  the  common  usage  which  sub- 
divides geology  into  a  number  of  departments,  —  dynamical, 
structural,  physiographic,  and  historical,  —  and  to  treat  in  im- 
mediate connection  with  each  geological  process  the  land  forms 
and  the  rock  structures  which  it  has  produced. 

It  is  hoped  that  the  facts  of  geology  and  the  inferences  drawn 
from  them  have  been  so  presented  as  to  afford  an  efficient  dis- 
cipline in  inductive  reasoning.  Typical  examples  have  been 
used  to  introduce  many  topics,  and  it  has  been  the  author's  aim 
to  give  due  proportion  to  both  the  wide  generalizations  of  our 
science  and  to  the  concrete  facts  on  which  they  rest. 

There  have  been  added  a  number  of  practical  exercises  such 
as  the  author  has  used  for  several  years  in  the  class  room. 
These  are  not  made  so  numerous  as  to  displace  the  problems 
which  no  doubt  many  teachers  prefer  to  have  their  pupils  solve 
impromptu  during  the  recitation,  but  may,  it  is  hoped,  suggest 
their  use. 

In  historical  geology  a  broad  view  is  given  of  the  develop- 
ment of  the  North  American  continent  and  the  evolution  of 

iii 


iv  PREFACE 

life  upon  the  planet.  Only  the  leading  types  of  plants  and 
animals  are  mentioned,  and  special  attention  is  given  to  those 
which  mark  the  lines  of  descent  of  forms  now  living. 

By  omitting  much  technical  detail  of  a  mineralogical  and 
paleontological  nature,  and  by  confining  the  field  of  view  almost 
wholly  to  our  own  continent,  space  has  been  obtained  to  give 
to  what  are  deemed  for  beginners  the  essentials  of  the  science 
a  fuller  treatment  than  perhaps  is  common. 

It  is  assumed  that  field  work  will  be  introduced  with  the 
commencement  of  the  study.  The  common  rocks  are  therefore 
briefly  described  in  the  opening  chapters.  The  drift  also  receives 
early  mention,  and  teachers  in  the  northern  states  who  begin 
geology  in  the  fall  may  prefer  to  take  up  the  chapter  on  the 
Pleistocene  immediately  after  the  chapter  on  glaciers. 

Simple  diagrams  have  been  used  freely,  not  only  because  they 
are  often  clearer  than  any  verbal  statement,  but  also  because 
they  readily  lend  themselves  to  reproduction  on  the  blackboard 
by  the  pupil.  The  text  will  suggest  others  which  the  pupil  may 
invent.  It  is  hoped  that  the  photographic  views  may  also  be 
used  for  exercises  in  the  class  room. 

The  generous  aid  of  many  friends  is  recognized  with  special 
pleasure.  To  Professor  W.  M.  Davis  of  Harvard  University  there 
is  owing  a  large  obligation  for  the  broad  conceptions  and  lumi- 
nous statements  of  geologic  facts  and  principles  with  which  he 
has  enriched  the  literature  of  our  science,  and  for  his  stimulating 
influence  in  education.  It  is  hoped  that  both  in  subject-matter 
and  in  method  the  book  itself  makes  evident  this  debt.  But 
besides  a  general  obligation  shared  by  geologists  everywhere,  and 
in  varying  degrees  by  perhaps  all  authors  of  recent  American 
text-books  in  earth  science,  there  is  owing  a  debt  direct  and 
personal.  The  plan  of  the  book,  with  its  use  of  problems  and 
treatment  of  land  forms  and  rock  structures  in  immediate  con- 
nection with  the  processes  which  produce  them,  was  submitted 


PREFACE  V 

to  Professor  Davis,  and,  receiving  Ms  approval,  was  carried  into 
effect,  although  without  the  sanction  of  precedent  at  the  time. 
Professor  Davis  also  kindly  consented  to  read  the  manuscript 
throughout,  and  his  many  helpful  criticisms  and  suggestions  are 
acknowledged  with  sincere  gratitude. 

Parts  of  the  manuscript  have  been  reviewed  by  Dr.  Samuel 
Calvin  and  Dr.  Frank  M.  Wilder  of  the  State  University  of 
Iowa ;  Dr.  S.  W.  Beyer  of  the  Iowa  College  of  Agriculture  and 
Mechanic  Arts;  Dr.  U.  S.  Grant  of  Northwestern  University; 
Professor  J.  A.  Udden  of  Augustana  College,  Illinois ;  Dr.  C.  H. 
Gordon  of  the  New  Mexico  State  School  of  Mines ;  Principal 
Maurice  Eicker  of  the  High  School,  Burlington,  Iowa ;  and  the 
following  former  students  of  the  author  who  are  engaged  in  the 
earth  sciences :  Dr.  W.  C.  Alden  of  the  United  States  Geological 
Survey  and  the  University  of  Chicago ;  Mr.  Joseph  Sniff  en,  in- 
structor in  the  Academy  of  the  University  of  Chicago,  Morgan 
Park ;  Professor  Martin  lorns,  Fort  Worth  University,  Texas ; 
Professor  A.  M.  Jayne,  Dakota  University;  Professor  G.  H. 
Bretnall,  Monmouth  College,  Illinois;  Professor  Howard  E. 
Simpson,  Colby  College,  Maine ;  Mr.  E.  J.  Cable,  instructor  in 
the  Iowa  State  Normal  College;  Principal  C.  C.  Gray  of  the 
High  School,  Fargo,  North  Dakota ;  and  Mr.  Charles  Persons  of 
the  High  School,  Hannibal,  Missouri.  A  large  number  of  the 
diagrams  of  the  book  were  drawn  by  Mr.  W.  W.  White  of  the 
Art  School  of  Cornell  College.  To  all  these  friends,  and  to 
the  many  who  have  kindly  supplied  the  illustrations  of  the  text, 
whose  names  are  mentioned  in  an  appended  list,  the  writer 
returns  his  heartfelt  thanks. 

WILLIAM  HARMON  NORTON 

CORNELL  COLLEGE,  MOUNT  VERNON,  IOWA 
July,  1906 


INTRODUCTORY  NOTE 

During  the  preparation  of  this  book  Professor  Norton  has  frequently 
discussed  its  plan  with  me  by  correspondence,  and  we  have  considered 
together  the  matters  of  scope,  arrangement,  and  presentation. 

As  to  scope,  the  needs  of  the  young  student  and  not  of  the  expert 
have  been  our  guide  ;  the  book  is  therefore  a  text-book,  not  a  reference 
volume. 

In  arrangement,  the  twofold  division  of  the  subject  was  chosen  be- 
cause of  its  simplicity  and  effectiveness.  The  principles  of  physical 
geology  come  first ;  the  several  chapters  are  arranged  in  what  is  believed 
to  be  a  natural  order,  appropriate  to  the  greatest  part  of  our  country, 
so  that  from  a  simple  beginning  a  logical  sequence  of  topics  leads 
through  the  whole  subject.  The  historical  view  of  the  science  comes 
second,  with  many  specific  illustrations  of  the  physical  processes  pre- 
viously studied,  but  now  set  forth  as  part  of  the  story  of  the  earth, 
with  its  many  changes  of  aspect  and  its  succession  of  inhabitants. 
Special  attention  is  here  given  to  North  America,  and  care  is  taken 
to  avoid  overloading  with  details. 

With  respect  to  method  of  presentation,  it  must  not  be  forgotten 
that  the  text-book  is  only  one  factor  in  good  teaching,  and  that  in 
geology,  as  in  other  sciences,  the  teacher,  the  laboratory,  and  the  local 
field  are  other  factors,  each  of  which  should  play  an  appropriate  part. 
The  text  suggests  observational  methods,  but  it  cannot  replace  observa- 
tion in  field  or  laboratory  ;  it  offers  certain  exercises,  but  space  cannot 
be  taken  to  make  it  a  laboratory  manual  as  well  as  a  book  for  study  ;  it 
explains  many  problems,  but  its  statements  are  necessarily  more  terse 
than  the  illustrative  descriptions  that  a  good  and  experienced  teacher 
should  supply.  Frequent  use  is  made  of  induction  and  inference  in 
order  that  the  student  may  come  to  see  how  reasonable  a  science  is 
geology,  and  that  he  may  avoid  the  too  common  error  of  thinking  that 
the  opinions  of  "authorities"  are  reached  by  a  private  road  that  is 
closed  to  him.  The  further  extension  of  this  method  of  presentation 
is  urged  upon  the  teacher,  so  that  the  young  geologist  may  always  learn 
the  evidence  that  leads  to  a  conclusion,  and  not  only  the  conclusion  itself. 

HARVARD  UNIVERSITY,  CAMBRIDGE,  MASS.  ^' 

July,  1905 


ACKNOWLEDGMENT  OF  ILLUSTRATIONS 

Adams,  Professor  F.  D.,  McGill  University,  Canada,  241. 

Alden,  Dr.  W.  C.,  Washington,  D.C.,  353. 

American  Museum  of  Natural  History,  New  York,  344. 

Ash,  H.  C.,  Galesburg,  111.,  133. 

Beyer,  Dr.  S.  W.,  Iowa  College  of  Agriculture,  363. 

Calvin,  Dr.  Samuel,  Iowa  State  University,  45,  295,  317,  325,  371. 

Carney,  Frank,  Ithaca,  N.Y.,  356. 

Clark,  Dr.  Wm.  B.,  Maryland  Geological  Survey,  43. 

Borne,  Dr.  Georg  v.  d.,  Jena,  Germany,  5,  6. 

Daly,  Dr.  R.  A.,  Ottawa,  Canada,  164. 

Defieux,  C.  A.,  Liverpool,  England,  154. 

*  Detroit  Photographic  Co.,  235,  236. 

*  Ellis,  W.  M.,  Edna,  Kan.,  13. 

Fairchild,  Professor  H.  L.,  University  of  Rochester,  141,  357. 

Field  Columbian  Museum,  Chicago,  87. 

Forster,  Dr.  A.  E.,  University  of  Vienna,  32. 

Gardner,  J.  L.,  Boston,  12,  140,  352. 

Geological  Survey  of  Canada,  256. 

Gilbert,  Dr.  G.  K.,  by  courtesy  of  the  American  Book  Company,  39. 

*  Haines,  Ben,  New  Albany,  Ind.,  33. 

*  Haynes,  F.  J.,  St.  Paul,  Minn.,  52,  95,  233. 
Henderson,  Judge  Julius,  Boulder,  Col.,  94. 

James,  George  Wharton,  Pasadena,  Cal.,  16,  127,  215,  229. 
Johnston-Lavis,  Professor  H.  J.,  Beaulieu,  France,  216. 
King,  J.  Harding,  Stourbridge,  England,  119. 
Lawson,  Dr.  Andrew  C.,  University  of  California,  113. 
Le  Conte,  Professor  J.  N.,  University  of  California,  8. 
Libbey,  Dr.  William,  Princeton  University,  92. 

*  McAllister,  T.  H.,  New  York,  242. 

*  Meyers,  H.  C.,  Boise,  Id.,  19. 

Mills,  Professor  H.  A.,  Cornell  College,  208,  304. 

vii 


viii  ACKNOWLEDGMENT  OF  ILLUSTRATIONS 

Norton,  Professor  W.  H.,  Cornell  College,  14,  35,  59,  88,  128,  183,  226, 
234,  255,  349,  364,  367. 

*  Notman,  Wm.  &  Son,  Montreal,  Canada,  98,  181. 
Obrutschew,  Dr.  W.,  Tomsk  Technological  Institute,  Siberia,  73. 
Oldham,  Dr.  R.  D.,  Geological  Survey  of  India,  120. 

*  Peabody,  H.  C.,  Pasadena,  Cal.,  54. 

*  Pierce,  C.  C.  &  Co.,  Los  Angeles,  Cal.,  15. 
Pillsbury,  Arthur,  San  Francisco,  Cal.,  115. 

*  Rau,  Wm.,  Philadelphia,  18,  21,  122,  123,  218. 
Reusch,  Dr.  Hans,  Geological  Survey  of  Norway,  112. 

Reynolds,  Professor  S.  H.,  University  College,  Bristol,  England,  202. 
Ricker,  Principal  Maurice,  Burlington,  Iowa,  48,  89. 

*  Shepard,  E.  A.,  Minneapolis,  Minn.,  105. 
Smith,  W.  S.  Tangier,  Los  Gatos,  Cal.,  186. 

*  Soule  Photographic  Co.,  Boston,  131. 

U.  S.  Geological  Survey,  3,  4,  23,  25,  34,  41,  63,  69,  78,  79,  80,  110, 
111,  114,  125,  126,  129,  130,  142,  151,  153,  169,  172,  177,  178, 
188,  211,  212,  214,  228,  237,  238,  239,  243,  244,  254,  257,  340, 
341,  353,  355. 

U.  S.  National  Museum,  149,  220,  221,  222,  225,  332. 

*  Valentine  &  Sons,  Dundee,  Scotland,  40,  136,  227. 
Vroman,  A.  C.,  Pasadena,  Cal.,  17. 

Ward's  Natural  Science  Establishment,  Rochester,  N.Y.,  152. 

*  Welch,  R.,  Belfast,  Ireland,  1,  37. 

Westgate,  Dr.  L.  G.,  Ohio  Wesleyan  University,  66. 
Whymper,  Edward,  London,  England,  106. 
Wilcox,  W.  D.,  Washington,  D.C.,  20. 
Wilson,  Dr.  A.  W.  G.,  McGill  University,  Canada,  68. 

*  Wilson,  G.  W.,  &  Co.,  Aberdeen,  Scotland,  82,  213. 

*  Worsley-Benisori,  F.  H.,  Cheapstow,  England,  170. 

*  Dealer  in  photographs  or  lantern  slides. 


CONTENTS 


PAGE 

INTRODUCTION THE  SCOPE  AND  AIM  OF  GEOLOGY      ...  1 

PART  I 

EXTERNAL  GEOLOGICAL  AGENCIES 
CHAPTER 

I.     THE  WORK  OF  THE  WEATHER 5 

II.     THE  WORK  OF  GROUND  WATER 39 

III.  RIVERS  AND  VALLEYS 54 

IV.  RIVER  DEPOSITS 93 

V.     THE  WORK  OF  GLACIERS 113 

VI.     THE  WORK  OF  THE  WIND 144 

VII.     THE  SEA  AND  ITS  SHORES 155 

VIII.     OFFSHORE  AND  DEEP-SEA  DEPOSITS 174 

PART  II 
INTERNAL  GEOLOGICAL  AGENCIES 

IX.     MOVEMENTS  OF  THE  EARTH'S  CRUST 195 

X.     EARTHQUAKES 233 

XI.     VOLCANOES 238 

XII.     UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN     .     .  265 

XIII.     METAMORPHISM  AND  MINERAL  VEINS    .  281 


x  CONTENTS 

PART  III 

HISTORICAL  GEOLOGY 

CHAPTER  PAGE 

XIV.     THE  GEOLOGICAL  RECORD 291 

XV.     THE  PRE-CAMBRIAN  SYSTEMS 304 

XVI.     THE  CAMBRIAN 315 

XVII.     THE  ORDOVICIAN  AND  SILURIAN 327 

XVIIT.     THE  DEVONIAN 341 

XIX.     THE  CARBONIFEROUS 350 

XX.     THE  MESOZOIC 368 

XXI.     THE  TERTIARY 394 

XXII.     THE  QUATERNARY 416 

INDEX  453 


.  THE 

ELEMENTS  OF  GEOLOGY 

INTRODUCTION 
THE  SCOPE  AND  AIM  OF  GEOLOGY 

Geology  deals  with  the  rocks  of  the  earth's  crust.  It  learns 
from  their  composition  and  structure  how  the  rocks  were  made 
and  how  they  have  been  modified.  It  ascertains  how  they  have 
been  brought  to  their  present  places  and  wrought  to  their  vari- 
ous topographic  forms,  such  as  hills  and  valleys,  plains  and 
mountains.  It  studies  the  vestiges  which  the  rocks  preserve  of 
ancient  organisms  which  once  inhabited  our  planet.  Geology  is 
the  history  of  the  earth  and  its  inhabitants,  as  read  in  the  rocks 
of  the  earth's  crust. 

To  obtain  a  general  idea  of  the  nature  and  method  of  our 
science  before  beginning  its  study  in  detail,  we  may  visit  some 
valley,  such  as  that  illustrated  in  the  frontispiece,  on  whose 
sides  are  rocky  ledges.  Here  the  rocks  lie  in  horizontal  layers. 
Although  only  their  edges  are  exposed,  we  may  infer  that  these 
layers  run  into  the  upland  on  either  side  and  underlie  the  entire 
district;  they  are  part  of  the  foundation  of  solid  rock  which 
everywhere  is  found  beneath  the  loose  materials  of  the  surface. 

The  ledges  of  the  valley  of  our  illustration  are  of  sandstone. 
Looking  closely  at  the  rock  we  see  that  it  is  composed  of 
myriads  of  grains  of  sand  cemented  together.  These  grains 
have  been  worn  and  rounded.  They  are  sorted  also,  those  of 
each  layer  being  about  of  a  size.  By  some  means  they  have 
been  brought  hither  from  some  more  ancient  source.  Surely 

1 


:£;•":  THE  ELEMENTS  OF  GEOLOGY 

these  grains  have  had  a  history  before  they  here  found  a  resting 
place,  —  a  history  which  we  are  to  learn  to  read. 

The  successive  layers  of  the  rock  suggest  that  they  were 
built  one  after  another  from  the  bottom  upward.  We  may  be  as 
sure  that  each  layer  was  formed  before  those  above  it  as  that 
the  bottom  courses  of  stone  in  a  wall  were  laid  before  the  courses 
which  rest  upon  them. 

We  have  no  reason  to  believe  that  the  lowest  layers  which 
we  see  here  were  the  earliest  ever  formed.  Indeed,  some  deep 
boring  in  the  vicinity  may  prove  that  the  ledges  rest  upon  other 
layers  of  rock  which  extend  downward  for  many  hundreds  of 
feet  below  the  valley  floor.  Nor  may  we  conclude  that  the 
highest  layers  here  were  the  latest  ever  laid ;  for  elsewhere  we 
may  find  still  later  layers  lying  upon  them. 

A  short  search  may  find  in  the  rock  relics  of  animals,  such 
as  the  imprints  of  shells,  which  lived  when  it  was  deposited; 
and  as  these  are  of  kinds  whose  nearest  living  relatives  now 
have  their  home  in  the  sea,  we  infer  that  it  was  on  the  flat  sea 
floor  that  the  sandstone  was  laid.  Its  present  position  hundreds 
of  feet  above  sea  level  proves  that  it  has  since  emerged  to  form 
part  of  the  land ;  while  the  flatness  of  the  beds  shows  that  the 
movement  was  so  uniform  and  gentle  as  not  to  break  or  strongly 
bend  them  from  their  original  attitude. 

The  surface  of  some  of  these  layers  is  ripple-marked.  Hence 
the  sand  must  once  have  been  as  loose  as  that  of  shallow  sea 
bottoms  and  sea  beaches  to-day,  which  is  thrown  into  similar 
ripples  by  movements  of  the  water.  In  some  way  the  grains 
have  since  become  cemented  into  firm  rock. 

Note  that  the  layers  on  one  side  of  the  valley  agree  with 
those  on  the  other,  each  matching  the  one  opposite  at  the  same 
level.  Once  they  were  continuous  across  the  valley.  Where  the 
valley  now  is  was  once  a  continuous  upland  built  of  horizontal 
layers ;  the  layers  now  show  their  edges,  or  outcrop,  on  the  valley 
sides  because  they  have  been  cut  by  the  valley  trench. 


THE  SCOPE  AND  AIM  OF  GEOLOGY  3 

The  rock  of  the  ledges  is  crumbling  away.  At  the  foot  of 
each  step  of  rock  lie  fragments  which  have  fallen.  Thus  the 
valley  is  slowly  widening.  It  has  been  narrower  in  the  past ;  it 
will  be  wider  in  the  future. 

Through  the  valley  runs  a  stream.  The  waters  of  rains  which 
have  fallen  on  the  upper  parts  of  the  stream's  basin  are  now  on 
their  way  to  the  river  and  the  sea.  Kock  fragments  and  grains 
of  sand  creeping  down  the  valley  slopes  come  within  reach  of 
the  stream  and  are  washed  along  by  the  running  water.  Here 
and  there  they  lodge  for  a  time  in  banks  of  sand  and  gravel, 
but  sooner  or  later  they  are  taken  up  again  and  carried  on.  The 
grains  of  sand  which  were  brought  from  some  ancient  source  to 
form  these  rocks  are  on  their  way  to  some  new  goal.  As  they 
are  washed  along  the  rocky  bed  of  the  stream  they  slowly  rasp 
and  wear  it  deeper.  The  valley  will  be  deeper  in  the  future ; 
it  has  been  less  deep  in  the  past. 

In  this  little  valley  we  see  slow  changes  now  in  progress.  We 
find  also  in  the  composition,  the  structure,  and  the  attitude  of  the 
rocks,  and  the  land  forms  to  which  they  have  been  sculptured, 
the  record  of  a  long  succession  of  past  changes  involving  the 
origin  of  sand  grains  and  their  gathering  and  deposit  upon  the 
bottom  of  some  ancient  sea,  the  cementation  of  their  layers  into 
solid  rock,  the  uplift  of  the  rocks  to  form  a  land  surface,  and, 
last  of  all,  the  carving  of  a  valley  in  the  upland. 

Everywhere,  in  the  fields,  along  the  river,  among  the  moun- 
tains, by  the  seashore,  and  in  the  desert,  we  may  discover  slow 
changes  now  in  progress  and  the  record  of  similar  changes  in  the 
past.  Everywhere  we  may  catch  gli mpses  of  a  process  of  gradual 
change,  which  stretches  backward  into  the  past  and  forward 
into  the  future,  by  which  the  forms  and  structures  of  the  face  of 
the  earth  are  continually  built  and  continually  destroyed.  The 
science  which  deals  with  this  long  process  is  geology.  Geology 
treats  of  the  natural  changes  now  taking  place  upon  the  earth 
and  within  it,  the  agencies  which  produce  them,  and  the  land 


4  THE  ELEMENTS  OF  GEOLOGY 

forms  and  rock  structures  which  result.  It  studies  the  changes 
of  the  present  in  order  to  be  able  to  read  the  history  of  the 
earth's  changes  in  the  past. 

The  various  agencies  which  have  fashioned  the  face  of  the 
earth  may  be  divided  into  two  general  classes.  In  Part  I  we 
shall  consider  those  which  work  upon  the  earth  from  without, 
such  as  the  weather,  running  water,  glaciers,  the  wind,  and  the 
sea.  In  Part  II  we  shall  treat  of  those  agencies  whose  sources  are 
within  the  earth,  and  among  whose  manifestations  are  volcanoes 
and  earthquakes  and  the  various  movements  of  the  earth's  crust. 
As  we  study  each  agency  we  shall  notice  not  only  how  it  does 
its  work,  but  also  the  records  which  it  leaves  in  the  rock  struc- 
tures and  the  land  forms  which  it  produces.  With  this  prepara- 
tion we  shall  be  able  in  Part  III  to  read  in  the  records  of  the 
rocks  the  history  of  our  planet  and  the  successive  forms  of  life 
which  have  dwelt  upon  it. 


PART  I 

EXTERNAL  GEOLOGICAL  AGENCIES 

CHAPTER  I* 
THE  WORK  OF  THE  WEATHER 

In  our  excursion  to  the  valley  with  sandstone  ledges  we  wit- 
nessed a  process  which  is  going  forward  in  all  lands.  Every- 
where the  rocks  are  crumbling  away ;  their  fragments  are  creep- 
ing down  hillsides  to  the  stream  ways  and  are  carried  by  the 
streams  to  the  sea,  where  they  are  rebuilt  into  rocky  layers. 
When  again  the  rocks  are  lifted  to  form  land  the  process  will 
begin  anew;  again  they  will  crumble  and  creep  down  slopes 
and  be  washed  by  streams  to  the  sea.  Let  us  begin  our  study 
of  this  long  cycle  of  change  at  the  point  where  rocks  disinte- 
grate and  decay  under  the  action  of  the  weather.  In  studying 
now  a  few  outcrops  and  quarries  we  shall  learn  a  little  of  some 
common  rocks  and  how  they  weather  away. 

Stratification  and  jointing.  At  the  sandstone  ledges  we  saw 
that  the  rock  was  divided  into  parallel  layers.  The  thicker 
layers  are  known  as  strata,  and  the  thin  leaves  into  which  each 
stratum  may  sometimes  be  split  are  termed  lamince.  To  a  greater 
or  less  degree  these  layers  differ  from  each  other  in  fineness  of 
grain,  showing  that  the  material  has  been  sorted.  The  planes 
which  divide  them  are  called  bedding  planes. 

Besides  the  bedding  planes  there  are  other  division  planes, 
which  cut  across  the  strata  from  top  to  bottom.  These  are 

5 


6 


THE  ELEMENTS  OF  GEOLOGY 


found  in  all  rocks  and  are  known  as  joints  (Fig.  1).  Two  sets 
of  joints,  running  at  about  right  angles  to  each  other,  together 
with  the  bedding  planes,  divide  the  sandstone  into  quadrangular 
blocks. 

Sandstone.    Examining  a  piece  of  sandstone  we  find  it  com- 
posed of  grains  quite  like  those  of  river  sand  or  of  sea  beaches. 

Most  of  the  grains 
are  of  a  clear  glassy 
mineral  called 
quartz.  These 
quartz  grains  are 
very  hard  and  will 
scratch  the  steel  of 
a  knife  blade.  They 
are  not  affected  by 
acid,  and  their 
broken  surfaces  are 
irregular  like  those 
of  broken  glass. 

The  grains  of 
sandstone  are  held 
together  by  some 
cement.  This  ma}7 
be  calcareous,  con- 
sisting of  soluble 
carbonate  of  lime. 
In  brown  sand- 
stones the  cement  is 
commonly  ferrugi- 
nous^—  hydrated  iron  oxide,  or  iron  rust,  forming  the  bond, 
somewhat  as  in  the  case  of  iron  nails  which  have  rusted  together. 
The  strongest  and  most  lasting  cement  is  siliceous,  and  sand 
rocks  whose  grains  are  closely  cemented  by  silica,  the  chemical 
substance  of  which  quartz  is  made,  are  known  as  quartzites. 


FIG.  1.   Cliff  of  Sandstone,  Ireland 

Note  the  horizontal  bedding  planes  and  the  two  sets 
of  vertical  joints  which  determine  the  cliff  faces 


THE  WORK  OF  THE  WEATHER 


We  are  now  prepared  to  understand  how  sandstone  is  affected 
by  the  action  of  the  weather.  On  ledges  where  the  rock  is 
exposed  to  view  its  surface  is  more  or  less,  discolored  and  the 
grains  are  loose  and  may  be  rubbed  off  with  the  finger.  On 
gentle  slopes  the  rock  is  covered  with  a  soil  composed  of  sand, 
which  evidently  is  crumbled  sandstone,  and  dark  carbonaceous 
matter  derived  from  the  decay  of  vegetation.  Clearly  it  is  by 
the  dissolving  of  the  cement  that  the  rock  thus  breaks  down  to 
loose  sand.  A  piece  of  sand- 
stone with  calcareous  cement, 
or  a  bit  of  old  mortar,  which 
is  really  an  artificial  stone 
also  made  of  sand  cemented 
by  lime,  may  be  treated  in  a 
test  tube  with  hydrochloric  FIG.  2.  Section  of  Limestone  Quarry  in 
acid  to  illustrate  the  process.  Southeastern  Wisconsin 

A  limestone  quarry.    Here     Scale,  1  in.  —  30  ft.     a,  red  residual  clay; 

inn,  pitted  surface  of  rotten  limestone ; 


66,  limestone  divided  into  thin  layers; 
c,  thick  layers  of  laminated  limestone, 
the  laminae  being  firmly  cemented 
together;  j,  j,  j,  joints.  Is  66  thin- 
layered  because  originally  so  laid,  or 
because  it  has  been  broken  up  by 
weathering,  although  once  like  c  thick- 
layered  ? 


also  we  find  the  rock  stratified 
and  jointed  (Fig.  2).  On  the 
quarry  face  the  rock  is  dis- 
tinctly seen  to  be  altered  for 
some  distance  from  its  upper 
surface.  Below  the  altered 
zone  the  rock  is  sound  and  is  quarried  for  building;  but  the 
altered  upper  layers  are  too  soft  and  broken  to  be  used  for  this 
purpose.  If  the  limestone  is  laminated,  the  laminse  here  have 
split  apart,  although  below  they  hold  fast  together.  Near  the 
surface  the  stone  has  become  rotten  and  crumbles  at  the  touch, 
while  on  the  top  it  has  completely  broken  down  to  a  thin  layer 
of  limestone  meal,  on  which  rests  a  fine  reddish  clay. 

Limestone  is  made  of  minute  grains  of  carbonate  of  lime  all 
firmly  held  together  by  a  calcareous  cement.  A  piece  of  the 
stone  placed  in  a  test  tube  with  hydrochloric  acid  dissolves 
with  brisk  effervescence,  leaving  the  insoluble  impurities,  which 


8  THE  ELEMENTS  OF   GEOLOGY. 

were  disseminated  through  it,  at  the  bottom  of  the  tube  as  a 
little  clay. 

We  can  now  understand  the  changes  in  the  upper  layers  of 
the  quarry.  At  the  surface  of  the  rock  the  limestone  has  com- 
pletely dissolved,  leaving  the  insoluble  residue  as  a  layer  of 
reddish  clay.  Immediately  below  the  clay  the  rock  has  dis- 
integrated into  meal  where  the  cement  between  the  limestone 
grains  has  been  removed,  while  beneath  this  the  laminae  are 
split  apart  where  the  cement  has  been  dissolved  only  along  the 
planes  of  lamination  where  the  stone  is  more  porous.  As  these 
changes  in  the  rock  are  greatest  at  the  surface  and  diminish 
downward,  we  infer  that  they  have  been  caused  by  agents 
working  downward  from  the  surface. 

At  certain  points  these  agencies  have  been  more  effective 
than  elsewhere.  The  upper  rock  surface  is  pitted.  Joints  are 
widened  as  they  approach  the  surface,  and  along  these  seams  we 
may  find  that  the  rock  is  altered  even  down  to  the  quarry  floor. 

A  shale  pit.  Let  us  now  visit  some  pit  where  shale  —  a 
laminated  and  somewhat  hardened  clay  —  is  quarried  for  the 
manufacture  of  brick.  The  laminae  of  this  fine-grained  rock  may 
be  as  thin  as  cardboard  in  places,  and  close  joints  may  break 
the  rock  into  small  rhombic  blocks.  On  the  upper  surface  we 
note  that  the  shale  has  weathered  to  a  clayey  soil  in  which 
all  traces  of  structure  have  been  destroyed.  The  clay  and  the 
upper  layers  of  the  shale  beneath  it  are  reddish  or  yellow,  while 
in  many  cases  the  color  of  the  unaltered  rock  beneath  is  blue. 

The  sedimentary  rocks.  The  three  kinds  of  layered  rocks 
whose  acquaintance  we  have  made  —  sandstone,  limestone,  and 
shale  —  are  the  leading  types  of  the  great  group  of  stratified,  or 
sedimentary,  rocks.  This  group  includes  all  rocks  made  of  sedi- 
ments, their  materials  having  settled  either  in  water  upon  the 
bottoms  of  rivers,  lakes,  or  seas,  or  on  dry  land,  as  in  the  case 
of  deposits  made  by  the  wind  and  by  glaciers.  Sedimentary 
rocks  are  divided  into  the  f ragmental  rocks  —  which  are  made 


THE  WORK  OF  THE  WEATHER 


9 


of  fragments,  either  coarse  or  fine  —  and  the  far  less  common 
rocks  which  are  constituted  of  chemical  precipitates. 

The  sedimentary  rocks  are  divided  according  to  their  com- 
position into  the  following  classes : 

1.  The  arenaceous,  or  quartz  rocks,  including  beds  of  loose 
sand  and  gravel,  sand- 
stone, quartzite,  and 

conglomerate  (a  rock 
made  of  cemented 
rounded  gravel  or  peb- 
bles). 

2.  The  calcareous,  or 
lime  rocks,  including 
limestone    and    a    soft 
white   rock    formed   of 
calcareous  powder 
known  as  chalk. 

3.  The   argillaceous, 
or  clay  rocks,  including 
muds,  clays,  and  shales. 

These  three  classes 
pass  by  mixture  into 
one  another.  Thus  FlG-'3-  Conglomerate 

there  are  limy  and  clayey  sandstones,  sandy  and  clayey  lime- 
stones, and  sandy  and  limy  shales. 

Granite.  This  familiar  rock  may  be  studied  as  an  example 
of  the  second  great  group  of  rocks,  —  the  unstratified,  or  igne- 
ous rocks.  These  are  not  made  of  cemented  sedimentary  grains, 
but  of  interlocking  crystals  which  have  crystallized  from  a  mol- 
ten mass.  Examining  a  piece  of  granite,  the  most  conspicuous 
crystals  which  meet  the  eye  are  those  of  feldspar.  They  are 
commonly  pink,  white,  or  yellow,  and  break  along  smooth  cleav- 
age planes  which  reflect  the  light  like  tiny  panes  of  glass. 
Mica  may  be  recognized  by  its  glittering  plates,  which  split  into 


10  THE  ELEMENTS  OF  GEOLOGY 

thin  elastic  scales.  A  third  mineral,  harder  than  steel,  break- 
ing along  irregular  surfaces  like  broken  glass,  we  identify  as 
quartz. 

How  granite  alters  under  the  action  of  the  weather  may 
be  seen  in  outcrops  where  it  forms  the  bed  rock,  or  country 
rock,  underlying  the  loose  formations  of  the  surface,  and  in 
many  parts  of  the  northern  states  where  granite  bowlders  and 
pebbles  more  or  less  decayed  may  be  found  in  a  surface  sheet 
of  stony  clay  called  the  drift.  Of  the  different  minerals  com- 
posing granite,  quartz  alone  remains  unaltered.  Mica  weathers 
to  detached  flakes  which  have  lost  their  elasticity.  The  feldspar 
crystals  have  lost  their  luster  and  hardness,  and  even  have  de- 
cayed to  clay.  Where  long-weathered  granite  forms  the  coun- 
try rock,  it  often  may  be  cut  with  spade  or  trowel  for  several 
feet  from  the  surface,  so  rotten  is  the  feldspar,  and  here  the  rock 
is  seen  to  break  down  to  a  clayey  soil  containing  grains  of  quartz 
and  flakes  of  mica. 

These  are  a  few  simple  illustrations  of  the  surface  changes 
which  some  of  the  common  kinds  of  rocks  undergo.  The  agen- 
cies by  which  these  changes  are  brought  about  we  will  now 
take  up  under  two  divisions,  —  chemical  agencies  producing 
rock  decay  and  mechanical  agencies  producing  rock  disinte- 
gration. 

THE  CHEMICAL  WORK  OF  WATER 

As  water  falls  on  the  earth  in  rain  it  has  already  absorbed 
from  the  air  carbon  dioxide  (carbonic  acid  gas)  and  oxygen.  As 
it  sinks  into  the  ground  and  becomes  what  is  termed  ground 
water,  it  takes  into  solution  from  the  soil  humus  acids  and 
carbon  dioxide,  both  of  which  are  constantly  being  generated 
there  by  the  decay  of  organic  matter.  So  both  rain  and  ground 
water  are  charged  with  active  chemical  agents,  by  the  help 
of  which  they  corrode  and  rust  and  decompose  all  rocks  to 
a  greater  or  less  degree.  We  notice  now  three  of  the  chief 


THE  WORK  OF  THE  WEATHER 


11 


chemical  processes  concerned  in  weathering,  —  solution,   the 
fOTmaUon_-pf  carbonates,  and  oxidation. 

Solution.  Limestone,  although  so  little  affected  by  pure  water 
that  five  thousand  gallons  would  be  needed  to  dissolve  a  sin- 
gle pound,  is  easily  dissolved  in  water  charged  with  carbon 
dioxide.  In  limestone  regions  well  water  is  therefore  "hard." 
On  boiling  the  water  for  some  time  the  carbon  dioxide  gas  is 


FIG.  4.   Surface  of  Limestone  furrowed  by  Weathering,  Montana 

expelled,  the  whole  of  the  lime  carbonate  can  no  longer  be  held 
in  solution,  and  much  of  it  is  thrown  down  to  form  a  crust  or 
"scale"  in  the  kettle  or  in  the  tubes  of  the  steam  boiler.  All 
waters  which  flow  over  limestone  rocks  or  soak  through  them 
are  constantly  engaged  in  dissolving  them  away,  and  in  the 
course  of  time  destroy  beds  of  vast  extent  and  great  thickness. 
The  upper  surface  of  limestone  rocks  becomes  deeply  pitted, 
as  we  saw  in  the  limestone  quarry,  and  where  the  mantle  of 
waste  has  been  removed  it  may  be  found  so  intricately  furrowed 
that  it  is  difficult  to  traverse  (Fig.  4). 


12  THE  ELEMENTS  OF  GEOLOGY 

Beds  of  rock  salt  buried  among  the  strata  are  dissolved  by 
seeping  water,  which  issues  in  salt  springs.  Gypsum,  a  mineral 
composed  of  hydr.ated  sulphate  of  lime,  and  so  soft  that  it  may 
be  scratched  with  the  finger  nail,  is  readily  taken  up  by  water, 
giving  to  the  water  of  wells  and  springs  a  peculiar  hardness 
difficult  to  remove. 

The  dissolving  action  of  moisture  may  be  noted  on  marble  tomb- 
stones of  some  age,  marble  being  a  limestone  altered  by  heat  and  pres- 
sure and  composed  of  crystalline  grains.  By  assuming  that  the  date  on 
each  monument  marks  the  year  of  its  erection,  one  may  estimate  how 
many  years  on  the  average  it  has  taken  for  weathering  to  loosen  fine 
grains  on  the  polished  surface,  so  that  they  may  be  rubbed  off  with  the 
finger,  to  destroy  the  polish,  to  round  the  sharp  edges  of  tool  marks  in 
the  lettering,  and  at  last  to  open  cracks  and  seams  and  break  down  the 
stone.  We  may  notice  also  whether  the  gravestones  weather  more 
rapidly  on  the  sunny  or  the  shady  side,  and  on  the  sides  or  on  the  top. 

The  weathered  surface  of  granular  limestone  containing  shells  shows 
them  standing  in  relief.  As  the  shells  are  made  of  crystalline  carbonate 
of  lime,  we  may  infer  whether  the  carbonate  of  lime  is  less  soluble  in 
its  granular  or  in  its  crystalline  condition. 

The  formation  of  carbonates.  In  attacking  minerals  water 
does  more  than  merely  take  them  into  solution.  It  decomposes 
them,  forming  new  chemical  compounds  of  which  the  carbonates 
are  among  the  most  important.  Thus  feldspar  consists  of  the 
insoluble  silicate  of  alumina,  together  witli  certain  alkaline 
silicates  which  are  broken  up  by  the  action  of  water  contain- 
ing carbon  dioxide,  forming  alkaline  carbonates.  These  carbon- 
ates are  freely  soluble  and  contribute  potash  and  soda  to  soils 
and  river  waters.  By  the  removal  of  the  soluble  ingredients  of 
feldspar  there  is  left  the  silicate  of  alumina,  united  with  water 
or  hydrated,  in  the  condition  of  a  fine  plastic  clay  which,  when 
white  and  pure,  is  known  as  kaolin  and  is  used  in  the  manu- 
facture of  porcelain.  Feldspathic  rocks  which  contain  no  iron 
compounds  thus  weather  to  whitish  crusts,  and  even  apparently 
sound  crystals  of  feldspar,  when  ground  to  thin  slices  and  placed 


THE  WORK  OF  THE  WEATHER  13 

under  the  microscope,  may  be  seen  to  be  milky  in  color  through- 
out because  an  internal  change  to  kaolin  has  begun. 

Oxidation.  Rocks  containing  compounds  of  iron  weather  to 
reddish  crusts,  and  the  seams  of  these  rocks  are  often  lined 
with  rusty  films.  Oxygen  and  water  have  here  united  with  the 
iron,  forming  hydrated  iron  oxide.  The  effects  of  oxidation  may 
be  seen  in  the  alteration  of 
many  kinds  of  rocks  and  in 
red  and  yellow  colors  of  soils 
and  subsoils. 


Pyrite  is  a  very  hard  mineral 
of  a  pale  brass  color,  found 
in  scattered  crystals  in  many 
rocks,  and  is  composed  of  iron 
and  sulphur  (iron  sulphide). 
Under  the  attack  of  the  weather 
it  takes  up  oxygen,  forming  FlG' 5"  Bowlder  split  by  Heat  and  Cold, 

'  ,L  .  ..  Western  Texas 

iron  sulphate  (green  vitriol),  a 

soluble  compound,  and  insoluble  hydrated  iron  oxide,  which  as  a 
mineral  is  known  as  limonite.  Several  large  masses  of  iron  sulphide 
were  placed  some  years  ago  on  the  lawn  in  front  of  the  National 
Museum  at  Washington.  The  mineral  changed  so  rapidly  to  green 
vitriol  that  enough  of  this  poisonous  compound  was  washed  into  the 
ground  to  kill  the  roots  of  the  surrounding  grass. 

AGENTS  OF  MECHANICAL  DISINTEGRATION 

Heat  and  cold.  Rocks  exposed  to  the  direct  rays  of  the  sun 
become  strongly  heated  by  day  and  expand.  After  sunset  they 
rapidly  cool  and  contract.  When  the  difference  in  temperature 
between  day  and  night  is  considerable,  the  repeated  strains  of 
sudden  expansion  and  contraction  at  last  become  greater  than 
the  rocks  can  bear,  and  they  break,  for  the  same  reason  that  a 
glass  cracks  when  plunged  into  boiling  water  (Fig.  5). 

Rocks  are  poor  conductors  of  heat,  and  hence  their  surfaces 
may  become  painfully  hot  under  the  full  blaze  of  the  sun,  while 


14 


THE  ELEMENTS  OF  GEOLOGY 


the  interior  remains  comparatively  cool.  By  day  the  surface 
shell  expands  and  tends  to  break  loose  from  the  mass  of  the 
stone.  In  cooling  in  the  evening  the  surface  shell  suddenly  con- 
tracts on  the  unyielding  interior  and  in  time  is  forced  off  in 
scales  (Fig.  6). 

Many  rocks,  such  as  granite,  are  made  up  of  grains  of  various 
minerals  which  differ  in  color  and  in  their  capacity  to  absorb 


FIG.  6.   Bowlders  scaling  off  under  Heat  and  Cold,  Western  Texas 

heat,  and  which  therefore  contract  and  expand  in  different 
ratios.  In  heating  and  cooling  these  grains  crowd  against  their 
neighbors  and  tear  loose  from  them,  so  that  finally  the  rock 
disintegrates  into  sand. 

The  conditions  for  the  destructive  action  of  heat  and  cold 
are  most  fully  met  in  arid  regions  when  vegetation  is  wanting 
for  lack  of  sufficient  rain.  The  soil  not  being  held  together 
by  the  roots  of  plants  is  blown  away  over  large  areas,  leav- 
ing the  rocks  bare  to  the  blazing  sun  in  a  cloudless  sky.  The 


THE  AVORK  OF  THE  WEATHER  15 

air  is  dry,  and  the  heat  received  by  the  earth  by  day  is  there- 
fore rapidly  radiated  at  night  into  space.  There  is  a  sharp 
and  sudden  fall  of  temperature  after  sunset,  and  the  rocks, 
strongly  heated  by  day,  are  now  chilled  perhaps  even  to  the 
freezing  point. 

In  the  Sahara  the  thermometer  has  been  known  to  fall  131°  F. 
within  a  few  hours.  In  the  light  air  of  the  Pamir  plateau  in  central 
Asia  a  rise  of  90°  F.  has  been  recorded  from  seven  o'clock  in  the 
morning  to  one  o'clock  in  the  afternoon.  On  the  mountains  of  south- 
western Texas  there  are  frequently  heard  crackling  noises  as  the  rocks 
of  that  arid  region  throw  off  scales  from  a  fraction  of  an  inch  to  four 
inches  in  thickness,  and  loud  reports  are  made  as  huge  bowlders  split 
apart.  Desert  pebbles  weakened  by  long  exposure  to  heat  and  cold  have 
been  shivered  to  fine  sharp-pointed  fragments  on  being  placed  in  sand 
heated  to  180°  F.  Beds  half  a  foot  thick,  forming  the  floor  of  lime- 
stone quarries  in  Wisconsin,  have  been  known  to  buckle  and  arch  and 
break  to  fragments  under  the  heat  of  the  summer  sun. 

Frost.  By  this  term  is  meant  the  freezing  and  thawing  of 
water  contained  in  the  pores  and  crevices  of  rocks.  All  rocks 
are  more  or  less  porous  and  all  contain  more  or  less  water 
in  their  pores.  Workers  in  stone  call  this  "quarry  water,"  and 
speak  of  a  stone  as  "  green  "  before  the  quarry  water  has  dried 
out.  Water  also  seeps  along  joints  and  bedding  planes  and 
gathers  in  all  seams  and  crevices.  Water  expands  in  freezing,  ten 
cubic  inches  of  water  freezing  to  about  eleven  cubic  inches  of 
ice.  As  water  freezes  in  the  rifts  and  pores  of  rocks  it  expands 
with  the  irresistible  force  illustrated  in  the  freezing  and  breaking 
of  water  pipes  in  winter.  The  first  rift  in  the  rock,  perhaps  too 
narrow  to  be  seen,  is  widened  little  by  little  by  the  wedges  of 
successive  frosts,  and  finally  the  rock  is  broken  into  detached 
blocks,  and  these  into  angular  chip-stone  by  the  same  process. 

It  is  on  mountain  tops  and  in  high  latitudes  that  the  effects  of  frost 
are  most  plainly  seen.  "  Every  summit,"  says  Whymper,  "  amongst  the 
rock  summits  upon  which  I  have  stood  has  been  nothing  but  a  piled-up 


16 


THE  ELEMENTS  OF  GEOLOGY 


heap  of  fragments  "  (Fig.  7).  In  Iceland,  in  Spitzbergen,  in  Kamchatka, 
and  in  other  frigid  lands  large  areas  are  thickly  strewn  with  sharp-edged 
fragments  into  which  the  rock  has  been  shattered  by  frost. 

Organic  agents.  We  must  reckon  the  roots  of  plants  and 
trees  among  the  agents  which  break  rocks  into  pieces.  The 
tiny  rootlet  in  its  search  for  food  and  moisture  inserts  itself 

into  some  minute 
rift,  and  as  it  grows 
slowly  wedges  the 
rock  apart. 

Moreover,  the 
acids  of  the  root  cor- 
rode the  rocks  with 
which  they  are  in 
contact.  One  may 
sometimes  find  in 
the  soil  a  block  of 
limestone  wrapped 


FIG.  7.   Rocks  broken  by  Frost,  Summit  of  the 
Eggischhorn,  Switzerland 


in  a  mesh  of  roots, 
each  of  which  lies  in  a  little  furrow  where  it  has  eaten  into 
the  stone. 

Bootless  plants  called  lichens  often  cover  and  corrode  rocks 
as  yet  bare  of  soil;  but  where  lichens  are  destroying  the  rock 
less  rapidly  than  does  the  weather,  they  serve  in  a  way  as  a 
protection. 

Conditions  favoring  disintegration  and  decay.  The  disinte- 
gration of  rocks  under  frost  and  temperature  changes  goes  on 
most  rapidly  in  cold  and  arid  climates,  and  where  vegetation 
is  scant  or  absent.  On  the  contrary,  the  decay  of  rocks  under 
the  chemical  action  of  water  is  favored  by  a  warm,  moist 
climate  and  abundant  vegetation.  Frost  and  heat  and  cold  can 
only  act  within  the  few  feet  from  the  surface  to  which  the 
necessary  temperature  changes  are  limited,  while  water  pene- 
trates and  alters  the  rocks  to  great  depths. 


THE  WORK  OF  THE  WEATHER  17 

The  pupil  may  explain 

In  what  ways  the  presence  of  joints  and  bedding  planes  assists  in  the 
breaking  up  and  decay  of  rocks  under  the  action  of  the  weather. 

Why  it  is  a  good  rule  of  stone  masons  never  to  lay  stones  on  edge, 
but  always  on  their  natural  bedding  planes. 

Why  stones  fresh  from  the  quarry  sometimes  go  to  pieces  in  early 
winter,  when  stones  which  have  been  quarried  for  some  months  remain 
uninjured. 

Why  quarrymen  in  the  northern  states  often  keep  their  quarry  floors 
flooded  during  winter. 

Why  laminated  limestone  should  not  be  used  for  curbstone. 

Why  rocks  composed  of  layers  differing  in  fineness  of  grain  and  in 
ratios  of  expansion  do  not  make  good  building  stone. 

Fine-grained  rocks  with  pores  so  small  that  capillary  attraction  keeps 
the  water  which  they  contain  from  readily  draining  away  are  more  apt  to 
hold  their  pores  ten  elevenths  full  of  water  than  are  rocks  whose  pores 
are  larger.  Wrhich,  therefore,  are  more  likely  to  be  injured  by  frost? 

Which  is  subject  to  greater  temperature  changes,  a  dark  rock  or  one 
of  a  light  color  ?  the  north  side  or  the  south  side  of  a  valley  ? 

THE  MANTLE  OF  ROCK  WASTE 

We  .have  seen  that  rocks  are  everywhere  slowly  wasting 
away.  They  are  broken  in  pieces  by  frost,  by  tree  roots,  and  by 
heat  and  cold.  They  dissolve  and  decompose  under  the  chemical 
action  of  water  and  the  various  corrosive  substances  which  it 
contains,  leaving  their  insoluble  residues  as  residual  clays  and 
sands  upon  the  surface.  As  a  result  there  is  everywhere  form- 
ing a  mantle  of  rock  waste  which  covers  the  land.  It  is  well  to 
imagine  how  the  country  would  appear  were  this  mantle  with 
its  soil  and  vegetation  all  scraped  away  or  had  it  never  been 
formed.  The  surface  of  the  land  would  then  be  everywhere  of 
bare  rock  as  unbroken  as  a  quarry  floor. 

The  thickness  of  the  mantle.  In  any  locality  the  thickness 
of  the  mantle  of  rock  waste  depends  as  much  on  the  rate  at 
which  it  is  constantly  being  removed  as  on  the  rate  at  which 


18  THE  ELEMENTS  OF  GEOLOGY 

it  is  forming.  On  the  face  of  cliffs  it  is  absent,  for  here  waste 
is  removed  as  fast  as  it  is  made.  Where  waste  is  carried  away 
more  slowly  than  it  is  produced,  it  accumulates  in  time  to  great 
depth. 

The  granite  of  Pikes  Peak  is  disintegrated  to  a  depth  of  twenty  feet. 
In  the  city  of  Washington  granite  rock  is  so  softened  to  a  depth  of  eighty 
feet  that  it  can  be  removed  with  pick  and  shovel.  About  Atlanta, 
Georgia,  the  rocks  are  completely  rotted  for  one  hundred  feet  from  the 
surface,  while  the  beginnings  of  decay  may  be  noticed  at  thrice  that 
depth.  In  places  in  southern  Brazil  the  rock  is  decomposed  to  a  depth 
of  four  hundred  feet. 

In  southwestern  Wisconsin  a  reddish  residual  clay  has  an  average 
depth  of  thirteen  feet  on  broad  uplands,  where  it  has  been  removed  to 
the  least  extent.  The  country  rock  on  which  it  rests  is  a  limestone 
with  about  ten  per  cent  of  insoluble  impurities.  At  least  how  thick, 
then,  was  that  portion  of  the  limestone  which  has  rotted  down  to 
the  clay? 

Distinguishing  characteristics  of  residual  waste.  We  must 
learn  to  distinguish  waste  formed  in  place  by  the  action  of  the 
weather  from  the  products  of  other  geological  agencies.  Resid- 
ual waste  is  unstratified.  It  contains  no  substances  which 
have  not  been  derived  from  the  weathering  of  the  parent  rock. 
There  is  a  gradual  transition  from  residual  waste  into  the 
un weathered  rock  beneath.  Waste  resting  on  sound  rock  evi- 
dently has  been  shifted  and  was  not  formed  in  place. 

In  certain  regions  of  southern  Missouri  the  land  is  covered  with  a 
layer  of  broken  flints  and  red  clay,  while  the  country  rock  is  limestone. 
The  limestone  contains  nodules  of  flint,  and  we  may  infer  that  it  has 
been  by  the  decay  and  removal  of  thick  masses  of  limestone  that  the 
residual  layer  of  clay  and  flints  has  been  left  upon  the  surface.  Flint  is 
a  form  of  quartz,  dull-lustered,  usually  gray  or  blackish  in  color,  and 
opaque  except  on  thinnest  edges,  where  it  is  translucent. 

Over  much  of  the  northern  states  there  is  spread  an  unstratified  stony 
clay  called  the  drift.  It  often  rests  on  sound  rocks.  It  contains  grains 
of  sand,  pebbles,  and  bowlders  composed  of  many  different  minerals  and 


THE  WORK  OF  THE  WEATHER  19 

rocks  that  the  country  rock  cannot  furnish.  Hence  the  drift  cannot 
have  been  formed  by  the  decay  of  the  rock  of  the  region.  A  shale  or 
limestone,  for  example,  cannot  waste  to  a  clay  containing  granite  peb- 
bles. The  origin  of  the  drift  will  be  explained  in  subsequent  chapters. 

The  differences  in  rocks  are  due  more  to  their  soluble  than  to 
their  insoluble  constituents.  The  latter  are  few  in  number  and 
are  much  the  same  in  rocks  of  widely  different  nature,  being 
chiefly  quartz,  silicate  of  alumina,  and  iron  oxide.  By  the 
removal  of  their  soluble  parts  very  many  and  widely  different 
rocks  rot  down  to  a  residual  clay  gritty  with  particles  of  quartz 
and  colored  red  or  yellow  with  iron  oxide. 

In  a  broad  way  the  changes  which  rocks  undergo  in  weather- 
ing are  an  adaptation  to  the  environment  in  which  they  find 
themselves  at  the  earth's  surface,  —  an  environment  different 
from  that  in  which  they  were  formed  under  sea  or  under  ground. 
In  open  air,  where  they  are  attacked  by  various  destructive 
agents,  few  of  the  rock-making  minerals  are  stable  compounds 
except  quartz,  the  iron  oxides,  and  the  silicate  of  alumina ;  and 
so  it  is  to  one  or  more  of  these  comparatively  insoluble  sub- 
stances that  most  rocks  are  reduced  by  long  decay. 

Which  produces  a  mantle  of  finer  waste,  frost  or  chemical  decay? 
which  a  thicker  mantle?  In  what  respects  would  you  expect  that  the 
mantle  of  waste  would  differ  in  warm  humid  lands  like  India,  in  frozen 
countries  like  Alaska,  and  in  deserts  such  as  the  Sahara? 

The  soil.  The  same  agencies  which  produce  the  mantle  of 
waste  are  continually  at  work  upon  it,  breaking  it  up  into  finer 
and  finer  particles  and  causing  its  more  complete  decay.  Thus 
on  the  surface,  where  the  waste  has  weathered  longest,  it  is 
gradually  made  fine  enough  to  support  the  growth  of  plants, 
and  is  then  known  as  soil.  The  coarser  waste  beneath  is  some- 
times spoken  of  as  subsoil.  Soil  usually  contains  more  or  less 
dark,  carbonaceous,  decaying  organic  matter,  called  humus,  and 
is  then  often  termed  the  humus  layer.  Soil  forms  not  only 
on  waste  produced  in  place  from  the  rock  beneath,  but  also  on 


20  THE  ELEMENTS  OF  GEOLOGY 

materials  which  have  been  transported,  such  as  sheets  of  glacial 
drift  and  river  deposits. 

Until  rocks  are  reduced  to  residual  clays  the  work  of  the  weather  is 
more  rapid  and  effective  on  the  fragments  of  the  mantle  of  waste  than 
on  the  rocks  from  which  waste  is  being  formed.  Why  ? 

Any  fresh  excavation  of  cellar  or  cistern,  or  cut  for  road  or 
railway,  will  show  the  characteristics  of  the  humus  layer.  It 
may  form  only  a  gray  film  011  the  surface,  or  we  may  find  it  a 
layer  a  foot  or  more  thick,  dark,  or  even  black,  above,  and  grow- 
ing gradually  lighter  in  color  as  it  passes  by  insensible  gradations 
into  the  subsoil.  In  some  way  the  decaying  vegetable  matter 
continually  forming  on  the  surface  has  become  mingled  with 
the  material  beneath  it. 

How  humus  and  the  subsoil  are  mingled.  The  mingling  of 
humus  and  the  subsoil  is  brought  about  by  several  means.  The 
roots  of  plants  penetrate  the  waste,  and  when  they  die  leave  their 
decaying  substance  to  fertilize  it.  Leaves  and  stems  falling  on 
the  surface  are  turned  under  by  several  agents.  Earthworms  and 
other  animals  whose  home  is  in  the  waste  drag  them  into  their 
burrows  either  for  food  or  to  line  their  nests.  Trees  overthrown 
by  the  wind,  roots  and  all,  turn  over  the  soil  and  subsoil  and 
mingle  them  together.  Bacteria  also  work  in  the  waste  and 
contribute  to  its  enrichment.  The  animals  living  in  the  mantle 
do  much  in  other  ways  toward  the  making  of  soil.  They  bring 
the  coarser  fragments  from  beneath  to  the  surface,  where  the 
waste  weathers  more  rapidly.  Their  burrows  allow  air  and 
water  to  penetrate  the  waste  more  freely  and  to  affect  it  to 
greater  depths. 

Ants.  In  the  tropics  the  mantle  of  waste  is  worked  over  chiefly  by 
ants.  They  excavate  underground  galleries  and  chambers,  extending 
sometimes  as  much  as  fourteen  feet  below  the  surface,  and  build  mounds 
which  may  reach  as  high  above  it.  In  some  parts  of  Paraguay  and 
southern  Brazil  these  mounds,  like  gigantic  potato  hills,  cover  tracts  of 
considerable  area. 


THE  WORK  OF  THE  WEATHER  21 

In  search  for  its  food  —  the  dead  wood  of  trees  —  the  so-called  white 
ant  constructs  runways  of  earth  about  the  size  of  gas  pipes,  reaching 
from  the  base  of  the  tree  to  the  topmost  branches.  On  the  plateaus  of 
central  Africa  explorers  have  walked  for  miles  through  forests  every 
tree  of  which  was  plastered  with  these  galleries  of  mud.  Each  grain  of 
earth  used  in  their  construction  is  moistened  and  cemented  by  slime 
as  it  is  laid  in  place  by  the  ant,  and  is  thus  acted  on  by  organic  chem- 
ical agents.  Sooner  or  later  these  galleries  are  beaten  down  by  heavy 
rains,  and  their  fertilizing  substances  are  scattered  widely  by  the  winds. 

Earthworms.  In  temperate  regions  the  waste  is  worked  over  largely 
by  earthworms.  In  making  their  burrows  worms  swallow  earth  in 
order  to  extract  from  it  any  nutritive  organic  matter  which  it  may 
contain.  They  treat  it  with  their  digestive  acids,  grind  it  in  their  stony 
gizzards,  and  void  it  in  castings  on  the  surface  of  the  ground.  It  was 
estimated  by  Darwin  that  in  many  parts  of  England  each  year,  on  every 
acre,  more  than  ten  tons  of  earth  pass  through  the  bodies  of  earthworms 
and  are  brought  to  the  surface,  and  that  every  few  years  the  entire  soil 
layer  is  thus  worked  over  by  them. 

In  all  these  ways  the  waste  is  niade  fine  and  stirred  and 
enriched.  Grain  by  grain  the  subsoil  with  its  fresh  mineral 
ingredients  is  brought  to  the  surface,  and  the  rich  organic 
matter  which  plants  and  animals  have  taken  from  the  atmos- 
phere is  plowed  under.  Thus  Nature  plows  and  harrows  on 
"the  great  world's  farm"  to  make  ready  and  ever  to  renew  a 
soil  fit  for  the  endless  succession  of  her  crops. 

The  world  processes  by  which  rocks  are  continually  wasting 
away  are  thus  indispensable  to  the  life  of  plants  and  animals. 
The  organic  world  is  built  on  the  ruins  of  the  inorganic,  and 
because  the  solid  rocks  have  been  broken  down  into  soil  men 
are  able  to  live  upon  the  earth. 

Solar  energy.  The  source  of  the  energy  which  accomplishes  all 
this  necessary  work  is  the  sun.  It  is  the  radiant  energy  of  the 
sun  which  causes  the  disintegration  of  rocks,  which  lifts  vapor 
into  the  atmosphere  to  fall  as  rain,  which  gives  life  to  plants 
and  animals.  Considering  the  earth  in  a  broad  way,  we  may 
view  it  as  a  globe  of  solid  rock, —  the  lithosphere, —  surrounded 


22  THE  ELEMENTS   OF   GEOLOGY 

by  two  mobile  envelopes :  the  envelope  of  air,  —  the  atmosphere , 
and  the  envelope  of  water,  —  the  hydrosphere.  Under  the 
action  of  solar  energy  these  envelopes  are  in  constant  motion. 
Water  from  the  hydrosphere  is  continually  rising  in  vapor  into 
the  atmosphere,  the  air  of  the  atmosphere  penetrates  the  hydro- 
sphere,—  for  its  gases  are  dissolved  in  all  waters,  —  and  both 
air  and  water  enter  and  work  upon  the  solid  earth.  By  their 
action  upon  the  lithosphere  they  have  produced  a  third  envelope, 
—  the  mantle  of  rock  waste. 

This  envelope  also  is  in  movement,  not  indeed  as  a  whole, 
but  particle  by  particle.  The  causes  which  set  its  particles  in 
motion,  and  the  different  forms  which  the  mantle  comes  to 
assume,  we  will  now  proceed  to  study. 

MOVEMENTS  OF  THE  MANTLE  OF  ROCK  WASTE 

At  the  sandstone  ledges  which  we  first  visited  we  saw  not 
only  that  the  rocks  were  crumbling  away,  but  also  that  grains 
and  fragments  of  them  were  creeping  down  the  slopes  of  the 
valley  to  the  stream  and  were  carried  by  it  onward  toward  the  sea. 
This  process  is  going  on  everywhere.  Slowly  it  may  be,  and 
with  many  interruptions,  but  surely,  the  waste  of  the  land  moves 
downward  to  the  sea.  We  may  divide  its  course  into  two 
parts,  —  the  path  to  the  stream,  which  we  will  now  consider, 
and  its  carriage  onward  by  the  stream,  which  we  will  defer  to 
a  later  chapter. 

Gravity.  The  chief  agent  concerned  in  the  movement  of 
waste  is  gravity.  Each  particle  of  waste  feels  the  unceasing 
downward  pull  of  the  earth's  mass  and  follows  it  when  free  to 
do  so.  All  agencies  which  produce  waste  tend  to  set  its  particles 
free  and  in  motion,  and  therefore  cooperate  with  gravity.  On 
cliffs,  rocks  fall  when  wedged  off  by  frost  or  by  roots  of  trees, 
and  when  detached  by  any  other  agency.  On  slopes  of  waste, 
water  freezes  in  chinks  between  stones,  and  in  pores  between 


THE  WORK  OF  THE  WEATHER  23 

particles  of  soil,  and  wedges  them  apart.  Animals  and  plants 
stir  the  waste,  heat  expands  it,  cold  contracts  it,  the  strokes  of 
the  raindrops  drive  loose  particles  down  the  slope  and  the  wind 
lifts  and  lets  them  fall.  Of  all  these  movements,  gravity  assists 
those  which  are  downhill  and  retards  those  which  are  uphill. 
On  the  whole,  therefore,  the  downhill  movements  prevail,  and 
the  mantle  of  waste,  block  by  block  and  grain  by  grain,  creeps 
along  the  downhill  path. 

A  slab  of  sandstone  laid  on  another  of  the  same  kind  at  an  angle  of 
17°  and  left  in  the  open  air  was  found  to  creep  down  the  slope  at 
the  rate  of  a  little  more  than  a  millimeter  a  month.  Explain  why  it 
did  so. 

Rain.  The  most  efficient  agent  in  the  carriage  of  waste  to 
the  streams  is  the  rain.  It  moves  particles  of  soil  by  the  force 
of  the  blows  of  the  falling  drops,  and  washes  them  down  all 
slopes  to  within  reach  of  permanent  streams.  On  surfaces  unpro- 
tected by  vegetation,  as  on  plowed  fields  and  in  arid  regions, 
the  rain  wears  furrows  and  gullies  both  in  the  mantle  of  waste 
and  in  exposures  of  unaltered  rock  (Fig.  17). 

At  the  foot  of  a  hill  we  may  find  that  the  soil  has  accumulated  by  creep 
and  wash  to  the  depth  of  several  feet ;  while  where  the  hillside  is  steepest 
the  soil  may  be  exceedingly  thin,  or  quite  absent,  because  removed 
about  as  fast  as  formed.  Against  the  walls  of  an  abbey  built  on  a  slope 
in  Wales  seven  hundred  years  ago,  the  creeping  waste  has  gathered  on  the 
uphill  side  to  a  depth  of  seven  feet.  The  slow-flowing  sheet  of  waste  is 
often  dammed  by  fences  and  walls,  whose  uphill  side  gathers  waste  in  a 
few  years  so  as  to  show  a  distinctly  higher  surface  than  the  downhill 
side,  especially  in  plowed  fields  .where  the  movement  is  least  checked 
by  vegetation. 

Talus.  At  tjie  foot  of  cliffs  there  is  usually  to  be  found  a 
slope  of  rock  fragments  which  clearly  have  fallen  from  above 
(Fig.  8).  Such  a  heap  of  waste  is  known  as  talus.  The  amount 
of  talus  in  any  place  depends  both  on  the  rate  of  its  formation 
and  the  rate  of  its  removal.  Talus  forms  rapidly  in  climates 


24 


THE  WORK  OF  THE  WEATHER  25 

where  mechanical  disintegration  is  most  effective,  where  rocks 
are  readily  broken  into  blocks  because  closely  jointed  and 
thinly  bedded  rather  than  massive,  and  where  they  are  firm 
enough  to  be  detached  in  fragments  of  some  size  instead  of  in 
fine  grains.  Talus  is  removed  slowly  where  it  decays  slowly, 
either  because  of  the  climate  or  the  resistance  of  the  rock.  It 
may  be  rapidly  removed  by  a  stream  flowing  along  its  base. 

In  a  moist  climate  a  soluble  rock,  such  as  massive  limestone, 
may  form  talus  little  if  any  faster  than  the  talus  weathers  away. 
A  loose-textured  sandstone  breaks  down  into  incoherent  sand 
grains,  which  in  dry  climates,  where  unprotected  by  vegetation, 
may  be  blown  away  as  fast  as.  they  fall,  leaving  the  cliff  bare  to 
fche  base.  Cliffs  of  such  slow-decaying  rocks  as  quartzite  and 
granite  when  closely  jointed  accumulate  talus  in  large  amounts. 

Talus  slopes  may  be  so  steep  as  to  reach  the  angle  of  repose, 
Le.  the  steepest  angle  at  which  the  material  will  lie.  This 
angle  varies  with  different  materials,  being  greater  with  coarse 
and  angular  fragments  than  with  fine  rounded  grains.  Sooner 
or  later  a  talus  reaches  that  equilibrium  where  the  amount 
removed  from  its  surface  just  equals  that  supplied  from  the 
cliff  above.  As  the  talus  is  removed  and 
weathers  away  its  slope  retreats  together 
with  the  retreat  of  the  cliff,  as  seen  in 
Figure  9. 

Graded  slopes.    Where  rocks  weather    FIG.  9.  Diagram  illustrat- 

faster  than  their  waste  is  carried  away,       ^  Retreat  of  Cliff,  c, 

/'         and  Talus,  t 
the  waste  comes  at  last  to  cover  all  rocky 

ledges.  On  the  steeper  slopes  it  is  coarser  and  in  more  rapid 
movement  than  on  slopes  more  gentle,  but  mountain  sides  and 
hills  and  plains  alike  come  to  be  mantled  with  sheets  of  waste 
which  everywhere  is  creeping  toward  the  streams.  Such  un- 
broken slopes,  worn  or  built  to  the  least  inclination  at  which 
the  waste  supplied  by  weathering  can  be  urged  onward,  are 
known  as  graded  slopes. 


26 


THE   ELEMENTS  OF   GEOLOGY 


Of  far  less  importance  than  the  silent,  gradual  creep  of  waste, 
which  is  going  on  at  all  times  everywhere  about  us,  are  the 
startling  local  and  spasmodic  movements  which  we  are  now  to 
describe. 

Avalanches.  On  steep  mountain  sides  the  accumulated  snows 
of  winter  often  slip  and  slide  in  avalanches  to  the  valleys  below. 


FIG.  10.  A  Landslide,  Quebec 

These  rushing  torrents  of  snow  sweep  their  tracks  clean  of 
waste  and  are  one  of  Nature's  normal  methods  of  moving  it 
along  the  downhill  path. 

Landslides.  Another  common  and  abrupt  method  of  deliver- 
ing waste  to  streams  is  by  slips  of  the  waste  mantle  in  large 
masses.  After  long  rains  and  after  winter  frosts  the  cohesion 
between  the  waste  and  the  sound  rock  beneath  is  loosened  bv 


THE  WORK  OF  THE  WEATHER  27 

seeping  water  underground.  The  waste  slips  on  the  rock  sur- 
face thus  lubricated  and  plunges  down  the  mountain  side  in  a 
swift  roaring  torrent  of  mud  and  stones. 

We  may  conveniently  mention  here  a  second  type  of  land- 
slide, where  masses  of  solid  rock  as  well  as  the  mantle  of  waste 
are  involved  in  the  sudden  movement. 
Such  slips  occur  when  valleys  have 
been  rapidly  deepened  by  streams  or 
glaciers  and  their  sides  have  not  yet 

been  graded.    A  favorable  condition  _ 

FIG.  11.  Diagram  illustrating 

is  where  the  strata  dip  (i.e.  incline       Conditions  favorable  to  a 
downwards)  towards  the  valley  (Fig.      Landslide 
11),  or  are   broken  by  joint  planes  im,  limestone  dipping  toward 
Clipping  in  the   Same   direction.     The        Bailey  of  river,  r ;  sh,  shale 

upper  layers,  including  perhaps  the  entire  mountain  side,  have 
been  cut  across  by  the  valley  trench  and  are  left  supported  only 
on  the  inclined  surface  of  the  underlying  rocks.  Water  may 
percolate  underground  along  this  surface  and  loosen  the  cohesion 
between  the  upper  and  the  underlying  strata  by  converting  the 
upper  surface  of  a  shale  to  soft  wet  clay,  by  dissolving  layers 
of  a  limestone,  or  by  removing  the  cement  of  a  sandstone  and 
converting  it  into  loose  sand.  When  the  inclined  surface  is 
thus  lubricated  the  overlying  masses  may  be  launched  into  the 
valley  below.  The  solid  rocks  are  broken  and  crushed  in  slid- 
ing and  converted  into  waste  consisting,  like  that  of  talus, 
of  angular  unsorted  fragments,  blocks  of  all  sizes  being  min- 
gled pellmell  with  rock  meal  and  dust.  The  principal  effects  of 
landslides  may  be  gathered  from  the  following  examples. 

At  Gohna,  India,  in  1893,  the  face  of  a  spur  four  thousand  feet  high, 
of  the  lower  ranges  of  the  Himalayas,  slipped  into  the  gorge  of  the 
headwaters  of  the  Ganges  River  in  successive  rock  falls  which  lasted  for 
three  days.  Blocks  of  stone  were  projected  for  a  mile,  and  clouds  of 
limestone  dust  were  spread  over  the  surrounding  country.  The  debris 
formed  a  dam  one  thousand  feet  high,  extending  for  two  miles  along  the 


28 


THE   ELEMENTS  OF   GEOLOGY 


valley.  A  lake  gathered  behind  this  barrier,  gradually  rising  until  it 
overtopped  it  in  a  little  less  than  a  year.  The  upper  portion  of  the 
dam  then  broke,  and  a  terrific  rush  of  water  swept  down  the  valley  in  a 
wave  which,  twenty  miles  away,  rose  one  hundred  and  sixty  feet  in  height. 
A  narrow  lake  is  still  held  by  the  strong  base  of  the  dam. 

In  1896,  after  forty  days  of  incessant  rain,  a  cliff  of  sandstone  slipped 
into  the  Yangtse  River  in  China,  reducing  the  width  of  the  channel  to 
eighty  yards  and  causing  formidable  rapids. 

At  Flims,  in  Switzerland,  a  prehistoric  landslip  flung  a  dam  eighteen 
hundred  feet  high  across  the  headwaters  of  the  Rhine.  If  spread 
evenly  over  a  surface  of  twenty-eight  square  miles,  the  material  would 

cover  it  to  a  depth  of  six 
hundred  and  sixty  feet. 
The  barrier  is  not  yet  en- 
tirely cut  away,  and  several 
lakes  are  held  in  shallow 
basins  on  its  hummocky 
surface. 

A  slide  from  the   pre- 
cipitous river  front  of  the 
citadel  hill  of  Quebec,  in 
1889,  dashed  across  Cham- 
plain    Street,    wrecking   a 
number  of  houses  and  caus- 
ing the  death  of  forty-five 
persons.    The   strata  here 
are    composed    of    steeply 
dipping  slate  (Fig.  10). 
In  lofty  mountain  ranges  there  may  not  be  a  single  valley  without 
its  traces  of  landslides,  so  common  there  is  this  method  of  the  movement 
of  waste,  and  of  building  to  grade  over-steepened  slopes. 


FIG.  12.   Bowlders  of   Weathering,    Granite 
Quarry,  Cape  Ann,  Massachusetts 

The  rock  is  divided  into  blocks  by  horizontal 
and  vertical  joint  planes.  How  do  the  bowl- 
ders of  the  upper  ledge  differ  in  shape  from 
those  beneath,  and  why  ? 


KOCK  SCULPTURE  BY  WEATHERING 

We  are  now  to  consider  a  few  of  the  forms  into  which  rock 
masses  are  carved  by  the  weather. 

Bowlders  of  weathering.  In  many  quarries  and  outcrops  we 
may  see  that  the  blocks  into  which  one  or  more  of  the  uppermost 


THE  WORK  OF  THE  WEATHER 


29 


layers  have  been  broken  along  their 
joints  and  bedding  planes  are  no 
longer  angular,  as  are  those  of  the 
layers  below.  The  edges  and  corners 
of  these  blocks  have  been  worn  away 
by  the  weather.  Such  rounded  cores, 
known  as  bowlders  of  weathering, 
are  often  left  to  strew  the  surface. 

Differential  weathering.    This 
term  covers  all  cases  in  which  a  rock    FIG  m  Differential  Weather. 
mass  weathers  differently  in  differ-        ing  on  a  Monument,  Colo- 
ent  portions.     Any  weaker  spots  or        T^° 
layers  are  etched  out  on  the  surface,  leaving  the  more  resistant 
in  relief.    Thus  massive  limestones  become  pitted  where  the 


FIG.  14.   Honeycombed  Limestone,  Iowa 

weather  drills  out  the  weaker  portions.  In  these  pits,  when 
once  they  are  formed,  moisture  gathers,  a  little  soil  collects, 
vegetation  takes  root,  and  thus  they  are  further  enlarged  until 
the  limestone  may  be  deeply  honeycombed. 


30 


THE  WORK  OF  THE  WEATHER  31 

On  the  sides  of  canyons,  and  elsewhere  where  the  edges  of 
strata  are  exposed,  the  harder  layers  project  as  cliffs,  while 
the  softer  weather  back  to  slopes  covered  with  the  talus  of  the 
harder  layers  above  them.  It  is  convenient  to  call  the  former 
cliff  makers  and  the  latter  slope  makers  (Fig.  15). 

Differential  weathering  plays  a  large  part  hi  the  sculpture 
of  the  land.  Areas  of  weak  rock  are  wasted  to  plains,  while 
areas  of  hard  rock  adjacent  are  still  left  as  hills  and  mountain 
ridges,  as  in  the  valleys  and  mountains  of  eastern  Pennsylvania. 
But  in  such  instances  the  lowering  of  the  surface  of  the  weaker 


.FIG.  16.    A  Small  Mesa,  New  Mexico 

rock  is  also  due  to  the  wear  of  streams,  and  especially  to  the 
removal  by  them  from  the  land  of  the  waste  which  covers  and 
protects  the  rocks  beneath. 

Rocks  owe  their  weakness  to  several  different  causes.  Some,  such  as 
beds  of  loose  sand,  are  soft  and  easily  worn  by  rains ;  some,  as  lime- 
stone and  gypsum  for  example,  are  soluble.  Even  hard  insoluble  rocks 
are  weak  under  the  attack  of  the  weather  when  they  are  closely  divided 
by  joints  and  bedding  planes  and  are  thus  readily  broken  up  into  blocks 
by  mechanical  agencies. 

Outliers  and  monuments.  As  cliffs  retreat  under  the  attack 
of  the  weather,  portions  are  left  behind  where  the  rock  is 
more  resistant  or  where  the  attack  for  any  reason  is  less  severe. 
Such  remnant  masses,  if  large,  are  known  as  outliers.  When 


THE  WORK  OF  THE  WEATHER 


33 


flat-topped,  because  of  the  protection  of  a  resistant  horizontal 
capping  layer,  they  are  termed  mesas  (Fig.  16),  —  a  term  applied 
also  to  the  flat-topped  portions  of  dissected  plateaus  (Fig.  129). 
Retreating  cliffs  may  fall  back  a  number  of  miles  behind  their 
outliers  before  the  latter  are  finally  consumed. 

Monuments  are  smaller  masses  and  may  be  but  partially 
detached  from  the  cliff  face.  In  the  breaking  down  of  sheets 
of  horizontal  strata,  outliers  grow  smaller  and  smaller  and  are 
reduced  to  massive  rectangular  monuments  resembling  castles 
(Fig.  17).  The  rock  castle  falls 
into  ruin,  leaving  here  and  there 
an  isolated  tower;  the  tower 
crumbles  to  a  lonely  pillar,  soon 
to  be  overthrown.  The  various 
and  often  picturesque  shapes  of 
monuments  depend  on  the  kind 
of  rock,  the  attitude  of  the 
strata,  and  the  agent  by  which 
they  are  chiefly  carved.  Thus 
pillars  may  have  a  capital  formed 
of  a  resistant  stratum.  Monu- 
ments may  be  undercut  and 
come  to  rest  on  narrow  pedes- 
tals, wherever  they  weather  more  rapidly  near  the  ground,  either 
because  of  the  greater  moisture  there,  or  —  in  arid  climates  — 
because  worn  at  their  base  by  drifting  sands. 

Stony  clays  disintegrating  under  the  rain  often  contain  bowlders 
which  protect  the  softer  material  beneath  from  the  vertical  blows  of 
raindrops,  and  thus  come  to  stand  on  pedestals  of  some  height 
(Fig.  19).  One  may  sometimes  see  on  the  ground  beneath  dripping  eaves 
pebbles  left  in  the  same  way,  protecting  tiny  pedestals  of  sand. 

Mountain  peaks  and  ridges.  Most  mountains  have  been  carved 
out  of  great  broadly  uplifted  folds  and  blocks  of  the  earth's 
crust.  Running  water  and  glacier  ice  have  cut  these  folds  and 


FIG.  18.   Undercut  Monuments, 
Colorado 


34 


THE  ELEMENTS  OF  GEOLOGY 


blocks  into  masses  divided  by  deep  valleys ;  but  it  is  by  the 
weather,  for  the  most  part,  that  the  masses  thus  separated  have 
been  sculptured  to  the  present  forms  of  the  individual  peaks 

and  ridges. 

Frost  and  heat  and  cold  sculpture 
high  mountains  to  sharp,  tusklike 
peaks  and  ragged,  serrate  crests, 
where  their  waste  is  readily  removed 
(Fig.  8). 

The  Matterhorn  of  the  Alps  is  a  fa- 
mous example  of  a  mountain  peak  whose 
carving  by  the  frost  and  other  agents  is 
in  active  progress.  On  its  face  "  scarcely 
a  rock  anywhere  is  firmly  attached,"  and 
the  fall  of  loosened  stones  is  incessant. 
Mountain  climbers  who  have  camped  at 
its  base  tell  how  huge  rocks  from  time 
to  time  come  leaping  down  its  precipices, 
followed  by  trains  of  dislodged  smaller 
fragments  and  rock  dust ;  and  how  at 
night  one  may  trace  the  course  of  the 
bowlders  by  the  sparks  which  they  strike 
from  the  mountain  walls.  Mount  Assini- 
boine,  Canada  (Fig.  20),  resembles  the 
Matterhorn  in  form  and  has  been  carved 
by  the  same  agencies. 

"  The  Needles "  of  Arizona  are  ex- 


FIG.  19. 


Roosevelt  Column. 
Idaho 


An  erosion  pillar  70  feet  high. 
How  was  it  produced?  Why 
quadrangular?  What  does  it  amples  of  sharp  mountain  peaks  in  a 
show  as  to  the  recent  height  of  warm  arid  region  sculptured  chiefly  by 
the  hillside  surf  ace? 

temperature  changes. 

Chemical  decay,  especially  when  carried  on  beneath  a  cover  of  waste 
arid  vegetation,  favors  the  production  of  rounded  knobs  and  dome-shaped 
mountains. 

The  weather  curve.  We  have  seen  that  weathering  reduces 
the  angular  block  quarried  by  the  frost  to  a  rounded  bowlder  by 
chipping  off  its  corners  and  smoothing  away  its  edges.  In  much 


THE   WORK  OF  THE  WEATHER 


35 


the  same  way  weathering  at  last  reduces  to  rounded  hills  the 
earth  blocks  cut  by  streams  or  formed  in  any  other  way.     High 


Mount  Assiniboine,  Canada 


mountains  may  at  first  be  sculptured  by  the  weather  to  savage 
peaks  (Fig.  181),  but  toward  the  end  of  their  life  history  they 


FIG.  21.   Big  Round  Top  and  Little  Round  Top, 
Gettysburg,  Pennsylvania 

wear  down  to  rounded  hills  (Fig.  182).  The  weather  curve, 
which  may  be  seen  on  the  summits  of  low  hills  (Fig.  21),  is 
convex  upward. 


36  THE  ELEMENTS  OF   GEOLOGY 

In  Figure  22,  representing  a  cubic  block  of  stone  whose  faces  are  a 
yard  square,  how  many  square  feet  of  surface  are  exposed  to  the 
weather  by  a  cubic  foot  at  a  corner  a ;  by  one  situated  in  the  middle 
of  an  edge  & ;  by  one  in  the  center  of  a  side  c  ?  How  much  faster 
will  a  and  b  weather  than  c,  and  what  will  be  the  effect  on  the  shape  of 
the  block  ? 

The  cooperation  of  various  agencies  in  rock  sculpture.   For 

the  sake  of  clearness  it  is  necessary  to  describe  the  work  of  each 
geological  agent  separately.  We  must  not  forget,  however,  that 
in  Nature  no  agent  works  independently  and  alone;  that  every 
result  is  the  outcome  of  a  long  chain  of  causes.  Thus,  in  order 
that  the  mountain  peak  may  be  carved  by 
the  agents  of  disintegration,  the  waste  must 
be  rapidly  removed, — a  work  done  by  many 
agents,  including  some  which  we  are  yet  to 
study ;  and  in  order  that  the  waste  may  be 
removed  as  fast  as  formed,  the  region  must 
first  have  been  raised  well  above  the  level  of 
the  sea,  so  that  the  agents  of  transportation  could  do  their  work 
effectively.  The  sculpture  of  the  rocks  is  accomplished  only  by 
the  cooperation  of  many  forces. 

The  constant  removal  of  waste  from  the  surface  by  creep 
and  wash  and  carriage  by  streams  is  of  the  highest  impor- 
tance, because  it  allows  the  destruction  of  the  land  by  means  of 
weathering  to  go  on  as  long  as  any  land  remains  above  sea 
level.  If  waste  were  not  removed,  it  would  grow  to  be  so  thick 
as  to  protect  the  rock  beneath  from  further  weathering,  and 
the  processes  of  destruction  which  we  have  studied  would  be 
brought  to  an  end.  The  very  presence  of  the  mantle  of  waste 
over  the  land  proves  that  on  the  whole  rocks  weather  more 
rapidly  than  their  waste  is  removed.  The  destruction  of  the 
land  is  going  on  as  fast  as  the  waste  can  be  carried  away. 

We  have  now  learned  to  see  in  the  mantle  of  waste  the 
record  of  the  destructive  action  of  the  agencies  of  weathering 


THE  WORK  OF  THE  WEATHER 


37 


on  the  rocks  of  the  land  surface.  Similar  records  we  shall  find 
buried  deeply  among  the  rocks  of  the  crust  in  old  soils  and  in 
rocks  pitted  and  decayed,  telling  of  old  land  surfaces  long 
wasted  by  the  weather.  Ever  since  the  dry  land  appeared  these 
agencies  have  been  as  now  quietly  and  unceasingly  at  work 
upon  it,  and  have  ever  been  the  chief  means  of  the  destruction 


FIG.  23.   Mount  Sneffels,  Colorado 

Describe  and  account  for  what  you  see  in  this  view.  What  changes  may 
the  mountain  be  expected  to  undergo  in  the  future  from  the  agencies 
now  at  work  upon  it  ? 

of  its  rocks.  The  vast  bulk  of  the  stratified  rocks  of  the  earth's 
crust  is  made  up  almost  wholly  of  the  waste  thus  worn  from 
ancient  lands. 

In  studying  the  various  geological  agencies  we  must  remem- 
ber the  almost  inconceivable  times  in  which  they  work.  The 
slowest  process  when  multiplied  by  the  immense  time  in  which 
it  is  carried  on  produces  great  results.  The  geologist  looks  upon 
the  land  forms  of  the  earth's  surface  as  monuments  which 
record  the  slow  action  of  weathering  and  other  agents  during 
the  ages  of  the  past.  The  mountain  peak,  the  rounded  hill,  the 


38  THE  ELEMENTS  OF  GEOLOGY 

wide  plain  which  lies  where  hills  and  mountains  once  stood, 
tell  clearly  of  the  great  results  which  slow  processes  will  reach 
when  given  long  time  in  which  to  do  their  work.  We  should 
accustom  ourselves  also  to  think  of  the  results  which  weather- 
ing will  sooner  or  later  bring  to  pass.  The  tombstone  and  the 
bowlder  of  the  field,  which  each  year  lose  from  their  surfaces 
a  few  crystalline  grains,  must  in  time  be  wholly  destroyed. 
The  hill  whose  rocks  are  slowly  rotting  underneath  a  cover  of 
waste  must  become  lower  and  lower  as  the  centuries  and  mil- 
lenniums come  and  go,  and  will  finally  disappear.  Even  the 
mountains  are  crumbling  away  continually,  and  therefore  are 
but  fleeting  features  of  the  landscape. 


CHAPTER  II 
THE  WORK  OF  GROUND  WATER 

Land  waters.  We  have  seen  how  large  is  the  part  that  water 
plays  at  and  near  the  surface  of  the  land  in  the  processes  of 
weathering  and  in  the  slow  movement  of  waste  down  all 
slopes  to  the  stream  ways.  We  now  take  up  the  work  of  water 
as  it  descends  beneath  the  ground,  —  a  corrosive  agent  still, 
and  carrying  in  solution  as  its  load  the  invisible  waste  of  rocks 
derived  from  their  soluble  parts. 

Land  waters  have  their  immediate  source  in  the  rainfall. 
By  the  heat  of  the  sun  water  is  evaporated  from  the  reservoir 
of  the  ocean  and  from  moist  surfaces  everywhere.  Mingled  as 
vapor  with  the  air,  it  is  carried  by  the  winds  over  sea  and  land, 
and  condensed  it  returns  to  the  earth  as  rain  or  snow.  That 
part  of  the  rainfall  which  descends  on  the  ocean  does  not  con- 
cern us,  but  that  which  falls  on  the  land  accomplishes,  as  it 
returns  to  the  sea,  the  most  important  work  of  all  surface 
geological  agencies. 

The  rainfall  may  be  divided  into  three  parts :  the  first  dries 
up,  being  discharged  into  the  air  by  evaporation  either  directly 
from  the  soil  or  through  vegetation ;  the  second  runs  off  over 
the  surface  to  flood  the  streams ;  the  third  soaks  in  the  ground 
and  is  henceforth  known  as  ground  or  underground  water. 

The  descent  of  ground  water.  Seeping  through  the  mantle 
of  waste,  ground  water  soaks  into  the  pores  and  crevices  of 
the  underlying  rock.  All  rocks  of  the  upper  crust  of  the 
are  more  or  less  porous,  and  all  drink  in  water.  Impervioud 
rocks,  such  as  granite,  clay,  and  shale,  have  pores  so  minute 
that  the  water  which  they  take  in  is  held  fast  within  them  by 


40  THE   ELEMENTS  OF   GEOLOGY 

capillary  attraction,  and  none  drains  through.  Pervious  rocks, 
on  the  other  hand,  such  as  many  sandstones,  have  pore  spaces 
so  large  that  water  niters  through  them  more  or  less  freely. 
Besides  its  seepage  through  the  pores  of  pervious  rocks,  water 
passes  to  lower  levels  through  the  joints  and  cracks  by  which 
all  rocks  near  the  surface  are  broken. 

Even  the  closest-grained  granite  has  a  pore  space  of  1  in  400,  while 
sandstone  may  have  a  pore  space  of  1  in  4.  Sand  is  so  porous  that  it 
may  absorb  a  third  of  its  volume  of  water,  and  a  loose  loam  even  as 
much  as  one  half. 

The  ground- water  surface  is  the  name  given  the  upper  surface 
of  ground  water,  the  level  below  which  all  rocks  are  saturated. 
In  dry  seasons  the  ground-water  surface  sinks.  For  ground 

water  is  constantly 
seeping    downward 
under  gravity,  it  is 
evaporated    in    the 
FIG.  24.   Diagram  illustrating  the  Relation  of  the    waste,  and  its  mois- 
Ground-Water  Surface  to  the  Surface  of  the    ture   ^    ^^ 
Ground 

ward  by  capillarity 

The  dotted  line  represents  the  ground-water  surface,  J  J 

and  arrows  indicate  the  direction  of  the  movements     and    the   roots   of 
of  ground  water,     m,  marsh ;  w,  well ;  r,  river  pknts  t()  the  gurf ace 

to  be  evaporated  in  the  air.  In  wet  seasons  these  constant 
losses  are  more  than  made  good  by  fresh  supplies  from  that 
part  of  the  rainfall  which  soaks  into  the  ground,  and  the  ground- 
water  surface  rises. 

In  moist  climates  the  ground-water  surface  (Fig.  24)  lies,  as 
a  rule,  within  a  few  feet  of  the  land  surface  and  conforms  to 
it  iii  a  general  way,  although  with  slopes  of  less  inclination 
than  those  of  the  hills  and  valleys.  In  dry  climates  permanent 
ground  water  may  be  found  only  at  depths  of  hundreds  of  feet. 
Ground  water  is  held  at  its  height  by  the  fact  that  its  circula- 
tion is  constantly  impeded  by  capillarity  and  friction.  If  it 
were  as  free  to  drain  away  as  are  surface  streams,  it  would 


THE  WORK  OF  GROUND  WATER 


41 


sink  soon  after  a  rain  to  the  level  of  the  deepest  valleys  of 
the  region. 

Wells  and  springs.  Excavations  made  in  permeable  rocks 
below  the  ground-water  surface  fill  to  its  level  and  are  known 
as  wells.  Where  valleys  cut  this  surface  permanent  streams  are 
formed,  the  water  either  oozing  forth  along  ill-defined  areas  or 
issuing  at  definite  points  called  springs,  where  it  is  concentrated 
by  the  structure  of  the 
rocks.  A  level  tract 
where  the  ground- water 
surface  coincides  with 
the  surface  of  the 
ground  is  a  swamp  or 
marsh. 

By  studying  a  spring 
one  may  learn  much  of 
the  ways  and  work  of 
ground  water.  Spring 
water  differs  from  that 
of  the  stream  into 
w^hich  it  flows  in  several 
respects.  If  we  test  the 
spring  with  a  ther- 
mometer during  succes- 
sive months,  we  shall  find  that  its  temperature  remains  much 
the  same  the  year  round.  In  summer  it  is  markedly  cooler  than 
the  stream ;  in  winter  it  is  warmer  and  remains  unfrozen  while 
the  latter  perhaps  is  locked  in  ice.  This  means  that  its  under- 
ground path  must  lie  at  such  a  distance  from  the  surface  that 
it  is  little  affected  by  summer's  heat  and  winter's  cold. 

While  the  stream  is  often  turbid  with  surface  waste  washed 
into  it  by  rains,  the  spring  remains  clear ;  its  water  has  been 
filtered  during  its  slow  movement  through  many  small  under- 
ground passages  and  the  pores  of  rocks.  Commonly  the  spring 


FIG.  25.   A  Spring,  Kansas 

Is  the  rock  over  which  the  spring  discharges 
pervious  or  impervious? 


42  THE  ELEMENTS  OF  GEOLOGY 

differs  from  the  stream  in  that  it  carries  a  far  larger  load  of 
dissolved  rock.  Chemical  analysis  proves  that  streams  contain 
various  minerals  in  solution,  but  these  are  usually  in  quantities 
so  small  that  they  are  not  perceptible  to  the  taste  or  feel.  But 
the  water  of  springs  is  often  well  charged  with  soluble  minerals  ; 
in  its  slow,  long  journey  underground  it  has  searched  out  the  sol- 
uble parts  of  the  rocks  through  which  it  seeps  and  has  dissolved 
as  much  of  them  as  it  could.  When  -spring  water  is  boiled  away, 
the  invisible  load  which  it  has  carried  is  left  behind,  and  in 
composition  is  found  to  be  practically  identical  with  that  of 
the  soluble  ingredients  of  the  country  rock.  Although  to  some 
extent  the  soluble  waste  of  rocks  is  washed  down  surface  slopes 
by  the  rain,  by  far  the  larger  part  is  carried  downward  by 
ground  water  and  is  delivered  to  streams  by  springs. 

In  limestone  regions  springs  are  charged  with  calcium  carbonate  (the 
carbonate  of  lime),  and  where  the  limestone  is  magnesian  they  contain 
magnesium  carbonate  also.  Such  waters  are  "  hard  ";  when  used  in  wash- 
ing, the  minerals  which  they  contain  combine  with  the  fatty  acids  of 
soap  to  form  insoluble  curdy  compounds.  When  springs  rise  from  rocks 
containing  gypsum  they  are  hard  with  calcium  sulphate.  In  granite 
regions  they  contain  more  or  less  soda  and  potash  from  the  decay  of 
feldspar. 

The  flow  of  springs  varies  much  less  during  the  different 
seasons  of  the  year  than  does  that  of  surface  streams.  So  slow 
is  the  movement  of  ground  water  through  the  rocks  that  even 
during  long  droughts  large  amounts  remain  stored  above  the 
levels  of  surface  drainage. 

Movements  of  ground  water.  Ground  water  is  in  constant 
movement  toward  its  outlets.  Its  rate  varies  according  to  many 
conditions,  but  always  is  extremely  slow.  Even  through  loose 
sands  beneath  the  beds  of  rivers  it  sometimes  does  not  exceed 
a  fifth  of  a  mile  a  year. 

In  any  region  two  zones  of  flow  may  be  distinguished.  The 
upper  zone  of  flow  extends  from  the  ground-water  surface 


THE  WORK  OF  GROUND  WATER 


43 


downward  through  the  waste  mantle  and  any  permeable  rocks 
on  which  the  mantle  rests,  as  far  as  the  first  impermeable  layer, 
where  the  descending  movement  of  the  water  is  stopped.  The 
deep  zones  of  flow 
occupy  any  pervi- 
ous rocks  which 
may  be  found  be- 
lt »w  the  impervious 
layer  which  lies 
nearest  to  the  sur- 


S 


FIG.  26.  Geological  Conditions  favorable  to 
Strong  Springs 


a,  limestone;  6,  shale;  c,  coarse  sandstone;  d,  lime- 
stone ;  e,  sandstone ;  /,  fissure.  The  strata  dip  toward 
the  south,  S.  Redraw  the  diagram,  marking  the  points 
at  which  strong  springs  (ss)  may  be  expected 


face.    The  upper 

zone  is  a  vast  sheet 

of  water  saturating 

the  soil  and  rocks  and  slowly  seeping  downward  through  their 

pores  and  interstices  along  the  slopes  to  the  valleys,  where  in 

part  it  discharges  in  springs  and  often  unites  also  in  a  wide 

underflowing  stream  which  supports  and  feeds  the  river  (Fig.  24). 


FIG.  28 
FIG.  27 

FIG.  27.   Diagram  of  Well  which  goes  dry  in  Drought,  a,  and  of  Unfailing 

Well,  b 

KM  raw  the  diagram,  showing  by  dotted  line  the  normal  ground-water  surface 
and  by  broken  line  the  ground-water  surface  at  times  of  drought 

Ku;.  28.   Diagram  of  Wet  Weather  Stream,  a,  and  of  Permanent  Stream,  6 
Redraw  the  diagram,  showing  ground-water  surface  by  dotted  line 

A  city  in  a  region  of  copious  rains,  built  on  the  narrow  flood  plain 
of  a  river,  overlooked  by  hills,  depends  for  its  water  supply  on  driven 
wells,  within  the  city  limits,  sunk  in  the  sand  a  few  yards  from  the  edge 
of  the  stream.  Are  these  wells  fed  by  water  from  the  river  percolating 
through  the  sand,  or  by  ground  water  on  its  way  to  the  stream  and 
possibly  contaminated  with  the  sewage  of  the  town  ? 


44  THE  ELEMENTS  OF   GEOLOGY 

At  what  height  does  underground  water  stand  in  the  wells  of  your 
region  ?  Does  it  vary  with  the  season  ?  Have  you  ever  known  wells  to 
go  dry  ?  It  may  be  possible  to  get  data  from  different  wells  and  to  draw 
a  diagram  showing  the  ground-water  surface  as  compared  with  the  sur- 
face of  the  ground. 

Fissure  springs  and  artesian  wells.  The  deeper  zones  of  flow 
lie  in  pervious  strata  which  are  overlain  by  some  impervious 
stratum.  Such  layers  are  often  carried  by  their  dip  to  great 
depths,  and  water  may  circulate  in  them  to  far  below  the  level 
of  the  surface  streams  and  even  of  the  sea.  When  a  fissure 
crosses  a  water-bearing  stratum,  or  aquifer,  water  is  forced 


FIG.  29.   Section  across  South  Dakota  from  the  Black  Hills  to  Sioux 
Falls  (S),  illustrating  the  Conditions  of  Artesian  Wells 

a,  crystalline  impervious  rocks;  6,  sedimentary  rocks,  shales,  limestones, 
and  sandstones;  c,  pervious  sandstone,  the  aquifer;  d,  impervious 
shales;  w,  w,  w,  artesian  wells 

upward  by  the  pressure  of  the  weight  of  the  water  contained  in 
the  higher  parts  of  the  stratum,  and  may  reach  the  surface  as  a 
fissure  spring.  A -boring  which  taps  such  an  aquifer  is  known 
as  an  artesian  well,  a  name  derived  from  a  province  in  France 
where  wells  of  this  kind  have  been  long  in  use.  The  rise  of 
the  water  in  artesian  wells,  and  in  fissure  springs  also,  depends 
on  the  folio  wing  conditions  illustrated  in  Figure  29.  The  aquifer 
dips  toward  the  region  of  the  wells  from  higher  ground,  where 
it  outcrops  and  receives  its  water.  It  is  inclosed  between  an 
impervious  layer  above  and  water-tight  or  water-logged  layers 
beneath.  The  weight  of  the  column  of  water  thus  inclosed  in 
the  aquifer  causes  water  to  rise  in  the  well,  precisely  as  the 
weight  of  the  water  in  a  standpipe  forces  it  in  connected  pipes 
to  the  upper  stories  of  buildings. 


THE  WORK  OF  GROUND  WATER  45 

Which  will  supply  the  larger  region  with  artesian  wells,  an  aquifer 
whose  dip  is  steep  or  one  whose  dip  is  gentle?  W^hich  of  the  two 
aquifers,  their  thickness  being  equal,  will  have  the  larger  outcrop  and 
therefore  be  able  to  draw  upon  the  larger  amount  of  water  from  the 
rainfall?  Illustrate  with  diagrams. 

The  zone  of  solution.  Near  the  surface,  where  the  circulation 
of  ground  water  is  most  active,  it  oxidizes,  corrodes,  and  dissolves 
the  rocks  through  which  it  passes.  It  leaches  soils  and  subsoils 
of  their  lime  and  other  soluble  minerals  upon  which  plants 
depend  for  their  food.  It  takes  away  the  soluble  cements  of 
rocks ;  it  widens  fissures  and  joints  and  opens  winding  passages 


FIG.  30.   Diagram  of  Caverns  and  Sink  Holes 

along  the  bedding  planes;  it  may  even  remove  whole  beds  of 
soluble  rocks,  such  as  rock  salt,  limestone,  or  gypsum.  The 
work  of  ground  water  in  producing  landslides  has  already  been 
noticed.  The  zone  in  which  the  work  of  ground  water  is  thus 
for  the  most  part  destructive  we  may  call  the  zone  of  solution. 
Caves.  In  massive  limestone  rocks,  ground  water  dissolves 
channels  which  sometimes  form  large  caves  (Fig.  30).  The 
necessary  conditions  for  the  excavation  of  caves  of  great  size 
are -well  shown  in  central  Kentucky,  where  an  upland  is  built 
throughout  of  thick  horizontal  beds  of  limestone.  The  absence 
of  layers  of  insoluble  or  impervious  rock  in  its  structure  allows 
a  free  circulation  of  ground  water  within  it  by  the  way  of  all 
natural  openings  in  the  rock.  These  water  ways  have  been  gradu- 
ally enlarged  by  solution  and  wear  until  the  upland  is  honey- 
combed with  caves.  Five  hundred  open  caverns  are  known  in 
one  county. 


46 


THE  ELEMENTS  OF  GEOLOGY 


Mammoth  Cave,  the  largest  of  these  caverns,  consists  of  a  labyrinth 
of  chambers  and  winding  galleries  whose  total  length  is  said  to  be 
as  much  as  thirty  miles.  One  passage  four  miles  long  has  an  average 
width  of  about  sixty  feet  and  an  average  height  of  forty  feet.  One  of 
the  great  halls  is  three  hundred  feet  in  width  and  is  overhung  by  a 
solid  arch  of  limestone  one  hundred  feet  above  the  floor.  Galleries  at 
different  levels  are  connected  by  well-like  pits,  some  of  which  measure 
two  hundred  and  twenty-five  feet  from  top  to  bottom.  Through  some  of 
the  lowest  of  these  tunnels  flows  Echo  River,  still  at  work  dissolving 
and  wearing  away  the  rock  while  on  its  dark  way  to  appear  at  the 
surface  as  a  great  spring. 

Natural  bridges.  As  a  cavern  enlarges  and  the  surface  of  the 
land  above  it  is  lowered  by  weathering,  the  roof  at  last  breaks 
down  and  the  cave  becomes  an  open  ravine.  A  portion  of  the 
roof  may  for  a  \vhile  remain,  forming  a  "  natural  bridge." 

Sink  holes.  In  limestone  regions  channels  under  ground  may 
become  so  well  developed  that  the  water  of  rains  rapidly  drains 

away  through  them. 
Ground  w  a  t  e  r 
stands  low  and  wells 
must  be  sunk  deep 
to  find  it.  Little  or 
no  surface  water  is 
left  to  form  brooks. 


Thus  across  the 
limestone  upland  of 
central  Kentucky  one 
meets  but  three  sur- 
face streams  in  a 


FIG.  31.   Sink  Holes  in  the  Karst,  Austria 


hundred  miles.  Between  their  valleys  surface  water  finds  its  way  under- 
ground by  means  of  sink  holes.  These  are  pits,  commonly  funnel 
shaped,  formed  by  the  enlargement  of  crevice  or  joint  by  percolating 
water,  or  by  the  breakdown  of  some  portion  of  the  roof  of  a  cave.  By 
clogging  of  the  outlet  a  sink  hole  may  come  to  be  filled  by  a  pond. 

Central  Florida  is  a  limestone  region  with  its  drainage  largely  sub- 
terranean and  in  part  below  the  level  even  of  the  sea.     Sink  holes  are 


THE  WORK  OF  GROUND  WATER 


47 


common,  and  many  of  them  are  occupied  by  lakelets.  Great  springs 
mark  the  point  of  issue  of  underground  streams,  while  some  rise  from 
beneath  the  sea.  Silver  Spring,  one  of  the  largest,  discharges  from  a 
basin  eight  hundred  feet  wide  and  thirty  feet  deep  a  little  river  navi- 
gable for  small  steamers  to  its  source.  About  the  spring  there  are  no 
surface  streams  for  sixty  miles. 

The  Karst.  Along  the  eastern  coast  of  the  Adriatic,  as  far  south  as 
Montenegro,  lies  a  belt  of  limestone  mountains  singularly  worn  and 
honeycombed  by  the  sol- 
vent  action  of  water. 
Where  forests  have  been 
cut  from  the  mountain 
sides  and  the  red  soil  has 
washed  away,  the  surface 
of  the  white  limestone 
forms  a  pathless  desert  of 
rock  where  each  square  rod 
has  been  corroded  into  an 
intricate  branch  work  of 
shallow  furrows  and  sharp 
ridges.  Great  sink  holes, 
some  of  them  six  hundred 
feet  deep  and  more,  pock- 
mark  the  surface  of  the 
land.  The  drainage  is 
chiefly  subterranean.  Sur- 
face streams  are  rare  and 
a  portion  of  their  courses 
is  often  under  ground. 
Fragmentary  valleys  come 
suddenly  to  an  end  at  walls 
of  rock  where  the  rivers  which  occupy  the  valleys  plunge  into  dark 
tunnels  to  reappear  some  miles  away.  Ground  water  stands  so  far 
below  the  surface  that  it  cannot  be  reached  by  wells,  and  the  inhabi- 
tants depend  on  rain  water  stored  for  household  uses.  The  finest 
cavern  of  Europe,  the  Adelsberg  Grotto,  is  in  this  region.  Karst,  the 
name  of  a  part  of  this  country,  is  now  used  to  designate  any  region 
or  landscape  thus  sculptured  by  the  chemical  action  of  surface  and 
ground  water.  We  must  remember  that  Karst  regions  are  rare,  and 


FIG.  32.   Underground  Stream  issuing  from 
Base  of  Cliff,  the  Karst,  Austria 


48 


THE  ELEMENTS  OF  GEOLOGY 


striking  as  is  the  work  of  their  subterranean  streams,  it  is  far  less 
important  than  the  work  done  by  the  sheets  of  underground  water 
slowly  seeping  through  all  subsoils  and  porous  rocks  in  other  regions. 
Even  when  gathered  into  definite  channels,  ground  water  does  not  have 
the  erosive  power  of  surface  streams,  since  it  carries  with  it  little  or  no 
rock  waste.  Regions  whose  underground  drainage  is  so  perfect  that  the 
development  of  surface  streams  has  been  retarded  or  prevented  escape 
to  a  large  extent  the  leveling  action  of  surface  running  waters,  and  may 
therefore  stand  higher  than  the  surrounding  country.  The  hill  honey- 
combed by  Luray  Cavern,  Virginia,  has  been  attributed  to  this  cause. 

Cavern  deposits.   Even  in  the  zone  of  solution  water  may  under 
certain  circumstances  deposit  as  well  as  erode.    As  it  trickles 

from  the  roof 
of  caverns,  the 
lime  carbonate 
which  it  has 
taken  into  so- 
lution from  the 
layers  of  lime- 
stone above  is 
deposited  by 
evaporation  in 
the  air  in  icicle- 
like  pendants 
called  stalac- 
tites. As  the 
drops  splash  on 
the  floor  there 

are  built  up  in  the  same  way  thicker  masses  called  stalagmites, 
which  may  grow  to  join  the  stalactites  above,  forming  pillars. 
A  stalagmitic  crust  often  seals  with  rock  the  earth  which 
accumulates  in  caverns,  together  with  whatever  relics  of  cave 
dwellers,  either  animals  or  men,  it  may  contain. 

Can  you  explain  why  slender  stalactites  formed  by  the  drip  of  single 
drops  are  often  hollow  pipes  ? 


FIG.  33.   Stalactites  and  Stalagmites,  Marengo 
Cavern,  Indiana 


THE  WORK  OF  GROUND  WATER  49 

The  zone  of  cementation.  With  increasing  depth  subterranean 
water  becomes  more  and  more  sluggish  in  its  movements  and 
more  and  more  highly  charged  with  minerals  dissolved  from 
the  rocks  above.  At  such  depths  it  deposits  these  minerals  in 
the  pores  of  rocks,  cementing  their  grains  together,  and  in 
crevices  and  fissures,  forming  mineral  veins.  Thus  below  the 
zone  of  solution  where  the  work  of  water  is  to  dissolve,  lies  the 
zone  of  cementation  where  its  work  is  chemical  deposit.  A  part 
of  the  invisible  load  of  waste  is  thus  transferred  from  rocks 
near  the  surface  to  those  at  greater  depths. 

As  the  land  surface  is  gradually  lowered  by  weathering  and 
the  work  of  rain  and  streams,  rocks  which  have  lain  deep  within 
the  zone  of  cementation  are  brought  within  the  zone  of  solution. 
Tims'  there  are  exposed  to  view  limestones,  whose  cracks  were 
filled  with  calcite  (crystallized  carbonate  of  lime),  with  quartz  or 
other  minerals,  and  sandstones  whose  grains  were  well  cemented 
many  feet  below  the  surface. 

Cavity  filling.  Small  cavities  in  the  rocks  are  often  found  more  or 
less  completely  filled  with  minerals  deposited  from  solution  by  water  in 
its  constant  circulation  underground.  The  process  may  be  illustrated 
by  the  deposit  of  salt  crystals  in  a  cup  of  evaporating  brine,  but  in  the 
latter  instance  the  solution  is  not  renewed  as  in  the  case  of  cavities  in 
the  rocks.  A  cavity  thus  lined  with  inward-pointing  crystals  is  called 
a  geode. 

Concretions.  Ground  water  seeping  through  the  pores  of  rocks  may 
gather  minerals  disseminated  throughout  them  into  nodular  masses 
called  concretions.  Thus  silica  disseminated  through  limestone  is 
gathered  into  nodules  of  flint.  While  geodes  grow  from  the  outside 
inwards,  concretions  grow  outwards  from  the  center.  Nor  are  they 
formed  in  already  existing  cavities  as  are  geodes.  In  soft  clays  con- 
cretions may,  as  they  grow,  press  the  clay  aside.  In  many  other  rooks 
concretions  are  made  by  the  process  of  replacement.  Molecule  by  mole- 
cule the  rock  is  removed  and  the  mineral  of  the  concretion  substituted 
in  its  place.  The  concretion  may  in  this  way  preserve  intact  the  lami- 
nation lines  or  other  structures  of  the  rock.  Clays  and  shales  often 


50 


THE  ELEMENTS  OF   GEOLOGY 


contain  concretions  of  lime  carbonate,  of  iron  carbonate,  or  of  iron 
sulphide.  Some  fossil,  such  as  a  leaf  or  shell,  frequently  forms  the 
nucleus  around  which  the  concretion  grows. 

Why  are  building  stones  more  easily  worked  when  "  green  "  than 
after  their  quarry  water  has  dried  out? 

Deposits  of  ground  water  in  arid  regions.    In  arid  lands  where 
ground  water  is  drawn  by  capillarity  to  the  surface  and  there 

evaporates,  it  leaves  as 
surface  incrustations  the 
minerals  held  in  solution. 
White  limy  incrusta- 
tions of  this  nature  cover 
considerable  tracts 
in  northern  Mexico. 
Evaporating  beneath  the 
surface,  ground  water 
may  deposit  a  limy 
cement  in  beds  of  loose 
sand  and  gravel.  Such 
firmly  cemented  layers 


FIG.  34.   Concretions  in  Sandstone, 
Wyoming 


are  not  uncommon   in  western  Kansas  and  Nebraska,  where 
they  are  known  as  "mortar  beds." 

Thermal  springs.  While  the  lower  limit  of  surface  drainage  is 
sea  level,  subterranean  water  circulates  much  below  that  depth, 
and  is  brought  again  to  the  surface  by  hydrostatic  pressure.  In 
many  instances  springs  have  a  higher  temperature  than  the 
average  annual  temperature  of  the  region,  and  are  then  known 
as  thermal  springs.  In  regions  of  present  or  recent  volcanic 
activity,  such  as  the  Yellowstone  National  Park,  we  may  believe 
that  the  heat  of  thermal  springs  is  derived  from  uncooled  lavas, 
perhaps  not  far  below  the 'surf  ace.  But  when  hot  springs  occur 
at  a  distance  of  hundreds  of  miles  from  any  volcano,  as  in  the 
case  of  the  hot  springs  of  Bath,  England,  it  is  probable  that 
their  waters  have  risen  from  the  heated  rocks  of  the  earth's 


THE  WORK  OF  GROUND  WATER 


51 


interior.  The  springs  of  Bath  have  a  temperature  of  120°  K, 
70°  above  the  average  annual  temperature  of  the  place.  If  we 
assume  that  the  rate  of  increase  in  the  earth's  internal  heat  is 
here  the  average  rate,  1°  F.  to  every  sixty  feet  of  descent,  we 
may  conclude  that  the  springs  of  Bath  rise  from  at  least  a  depth 
of  forty-two  hundred  feet. 

Water  may  descend  to  depths  from  which  it  can  never  be 
brought  back  by  hydrostatic  pressure.  It  is  absorbed  by  highly 
heated  rocks  deep  below  the  surface.  From  time  to  time  some 
of  this  deep-seated  water  may  be  returned  to  open  air  in  the 
steam  of  volcanic  eruptions. 


FIG.  35.  Calcareous  Deposits  from  Hot  Springs,  Yellowstone 
National  Park 

Surface  deposits  of  springs.  Where  subterranean  water  re- 
turns to  the  surface  highly  charged  with  minerals  in  solution,  on 
exposure  to  the  air  it  is  commonly  compelled  to  lay  down  much 
of  its  invisible  load  in  chemical  deposits  about  the  spring.  These 
are  thrown  down  from  solution  either  because  of  cooling,  evap- 
oration, the  loss  of  carbon  dioxide,  or  the  work  of  algae. 

Many  springs  have  been  charged  under  pressure  with  carbon 
dioxide  from  subterranean  sources  and  are  able  therefore  to 


52  THE  ELEMENTS  OF  GEOLOGY 

take  up  large  quantities  of  lime  carbonate  from  the  limestone 
rocks  through  which  they  pass.  On.  reaching  the  surface  the 
pressure  is  relieved,  the  gas  escapes,  and  the  lime  carbonate  is 
thrown  down  in  deposits  called  travertine.  The  gas  is  some- 
times withdrawn  and  the  deposit  produced  in  large  part  by  the 
action  of  algae  and  other  humble  forms  of  plant  life. 

At  the  Mammoth  Hot  Springs  in  the  valley  of  the  Gardiner  River, 
Yellowstone  National  Park,  beautiful  terraces  and  basins  of  travertine 
(Fig.  35)  are  now  building,  chiefly  by  means  of  algae  which  cover  the 
bottoms,  rims,  and  sides  of  the  basins  and  deposit  lime  carbonate  upon 
them  in  successive  sheets.  The  rock,  snow-white  where  dry,  is  coated 
with  red  and  orange  gelatinous  mats  where  the  algse  thrive  in  the  over- 
flowing waters. 

Similar  terraces  of  travertine  are  found  to  a  height  of  fourteen 
hundred  feet  up  the  valley  side.  We  may  infer  that  the  springs  which 
formed  these  ancient  deposits  discharged  near  what  was  then  the  bottom 
of  the  valley,  and  that  as  the  valley  has  been  deepened  by  the  river  the 
ground  water  of  the  region  has  found  lower  and  lower  points  of  issue. 

In  many  parts  of  the  country  calcareous  springs  occur  which  coat 
with  lime  carbonate  mosses,  twigs,  and  other  objects  over  which  their 
waters  flow.  Such  are  popularly  known  as  petrifying  springs,  although 
they  merely  incrust  the  objects  and  do  not  convert  them  into  stone. 

Silica  is  soluble  in  alkaline  waters,  especially  when  these  are 
hot.  Hot  springs  rising  through  alkaline  siliceous  rocks,  such  as 
lavas,  often  deposit  silica  in  a  white  spongy  formation  known 
as  siliceous  sinter,  both  by  evaporation  and  by  the  action  of 
algae  which  secrete  silica  from  the  waters.  It  is  in  this  way  that 
the  cones  and  mounds  of  the  geysers  in  the  Yellowstone  National 
Park  and  in  Iceland  have  been  formed  (Fig.  234). 

Where  water  oozes  from  the  earth  one  may  sometimes  see 
a  rusty  deposit  on  the  ground,  and  perhaps  an  iridescent  scum 
upon  the  water.  The  scum  is  often  mistaken  for  oil,  but  at  a 
touch  it  cracks  and  breaks,  as  oil  would  not  do.  It  is  a  film  of 
bydrated  iron  oxide,  or  limonite,  and  the  spring  is  an  iron,  or 
chalybeate,  spring.  Compounds  of  iron  have  been  taken  into 


THE  WORK  OF  GROUND  WATER  53 

solution  by  ground  water  from  soil  and  rocks,  and  are  now 
changed  to  the  insoluble  oxide  on  exposure  to  the  oxygen  of 
the  air. 

In  wet  ground  iron  compounds  leached  by  ground  water  from  the 
soil  often  collect  in  reddish  deposits  a  few  feet  below  the  surface,  where 
their  downward  progress  is  arrested  by  some  impervious  clay.  At  the 
bottom  of  bogs  and  shallow  lakes  iron  ores  sometimes  accumulate  to  a 
depth  of  several  feet. 

Decaying  organic  matter  plays  a  large  part  in  these  changes.  In  its 
presence  the  insoluble  iron  oxides  which  give  color  to  most  red  and 
yellow  rocks  are  decomposed,  leaving  the  rocks  of  a  gray  or  bluish  color, 
and  the  soluble  iron  compounds  which  result  are  readily  leached  out,  — 
effects  seen  where  red  or  yellow  clays  have  been  bleached  about  some 
decaying  tree  root. 

The  iron  thus  dissolved  is  laid  down  as  limonite  when  oxidized,  as 
about  a  chalybeate  spring ;  but  out  of  contact  with  the  air  and  in  the 
presence  of  carbon  dioxide  supplied  by  decaying  vegetation,  as  in  a  peat 
bog,  it  may  be  deposited  as  iron  carbonate,  or  siderite. 

Total  amount  of  underground  waters.  In  order  to  realize 
the  vast  work  in  solution  and  cementation  which  underground 
waters  are  now  doing  and  have  done  in  all  geological  ages, 
we  must  gain  some  conception  of  their  amount.  At  a  certain 
depth,  estimated  at  about  six  miles,  the  weight  of  the  crust  be- 
comes greater  than  the  rocks  can  bear,  and  all  cavities  and  pores 
in  them  must  be  completely  closed  by  the  enormous  pressure 
which  they  sustain.  Below  a  depth  of  even  three  or  four  miles 
it  is  believed  that  ground  water  cannot  circulate.  Estimating 
the  average  pore  spaces  of  the  different  rocks  of  the  earth's  crust 
above  this  depth,  and  the  average  per  cents  of  their  pore  spaces 
occupied  by  water,  it  has  been  recently  computed  that  the  total 
amount  of  ground  water  is  equal  to  a  sheet  of  water  one  hundred 
feet  deep,  covering  the  entire  surface  of  the  earth. 


CHAPTEE   III 
RIVERS  AND  VALLEYS 

The  run-off.  We  have  traced  the  history  of  that  portion  of 
the  rainfall  which  soaks  into  the  ground ;  let  us  now  return  to 
that  part  which  washes  along  the  surface  and  is  known  as  the 
run-off.  Fed  by  rains  and  melting  snows,  the  run-off  gathers 
into  courses,  perhaps  but  faintly  marked  at  first,  which  join 
more  definite  and  deeply  cut  channels,  as  twigs  their  stems.  In 
a  humid  climate  the  larger  ravines  through  which  the  run-off 
flows  soon  descend  below  the  ground-water  surface.  Here 
springs  discharge  along  the  sides  of  the  little  valleys  and  per- 
manent streams  begin.  The  water  supplied  by  the  run-off  here 
joins  that  part  of  the  rainfall  which  had  soaked  into  the  soil, 
and  both  now  proceed  together  by  way  of  the  stream  to  the  sea. 

River  floods.  Streams  vary  greatly  in  volume  during  the  year. 
At  stages  of  flood  they  fill  their  immediate  banks,  or  overrun 
them  and  inundate  any  low  lands  adjacent  to  the  channel ;  at 
stages  of  low  water  they  diminish  to  but  a  fraction  of  their  vol- 
ume when  at  flood. 

At  times  of  flood,  rivers  are  fed  chiefly  by  the  run-off;  at 
times  of  low  water,  largely  or  even  wholly  by  springs. 

How,  then,  will  the  water  of  streams  differ  at  these  times  in  tur- 
bidity and  in  the  relative  amount  of  solids  carried  in  solution? 

In  parts  of  England  streams  have  been  known  to  continue  flowing 
after  eighteen  months  of  local  drought,  so  great  is  the  volume  of  water 
which  in  humid  climates  is  stored  in  the  rocks  above  the  drainage  level, 
and  so  slowly  is  it  given  off  in  springs. 

In  Illinois  and  the  states  adjacent,  rivers  remain  low  in  winter  and  a 
"  spring  freshet "  follows  the  melting  of  the  winter's  snows.  A  "  June 

64 


RIVERS  AND  VALLEYS  55 

rise  "  is  produced  by  the  heavy  rains  of  early  summer.  Low  water  fol- 
lows in  July  and  August,  and  streams  are  again  swollen  to  a  moderate 
degree  under  the  rains  of  autumn. 

The  discharge  of  streams.  The  per  cent  of  rainfall  discharged 
by  rivers  varies  with  the  amount  of  rainfall,  the  slope  of  the 
drainage  area,  the  texture  of  the  rocks,  and  other  factors.  With 
an  annual  rainfall  of  fifty  inches  hi  an  open  country,  about  fifty 
per  cent  is  discharged ;  while  with  a  rainfall  of  twenty  inches 
only  fifteen  per  cent  is  discharged,  part  of  the  remainder  being 
evaporated  and  part  passing  underground  beyond  the  drainage 
area.  Thus  the  Ohio  discharges  thirty  per  cent  of  the  rainfall  of 
its  basin,  while  the  Missouri  carries  away  but  fifteen  per  cent. 
A  number  of  the  streams  of  the  semi-arid  lands  of  the  West  do 
not  discharge  more  than  five  per  cent  of  the  rainfall. 

Other  things  being  equal,  which  will  afford  the  larger  proportion  of 
run-off,  a  region  underlain  with  granite  rock  or  with  coarse  sandstone  ? 
grass  land  or  forest  ?  steep  slopes  or  level  land  ?  a  well-drained  region 
or  one  abounding  in  marshes  and  ponds  ?  frozen  or  unfrozen  ground  ? 
Will  there  be  a  larger  proportion  of  run-off  after  long  rains  or  after 
a  season  of  drought  ?  after  long  and  gentle  rains,  or  after  the  same 
amount  of  precipitation  in  a  violent  rain?  during  the  months  of  grow- 
ing vegetation,  from  June  to  August,  or  during  the  autumn  months? 

Desert  streams.  In  arid  regions  the  ground-water  surface  lies 
so  low  that  for  the  most  part  stream  ways  do  not  intersect  it. 
Streams  therefore  are  not  fed  by  springs, 
but  instead  lose  volume  as  their  waters 

soak  into  the  thirsty  rocks   over  which       ' 

they  flow.    They  contribute  to  the  ground    FIG.  36.  Rise  of  Ground- 
water  of  the  region  instead  of  being  in-      Water  Surface  (broken 
.   _       .        °    .  ..    ,     ,  .    _ °    ,  line)   beneath   Valley 

creased  by  it.    Being  supplied  chiefly  by       (F)  in  Arid  Region 

the  run-off,  they  wither  at  times  of  drought 

to  a  mere  trickle  of  water,  to  a  chain  of  pools,  or  go  wholly 
dry,  while  at  long  intervals  rains  fill  their  dusty  beds  with 
sudden  raging  torrents.  Desert  rivers  therefore  periodically 


56  THE  ELEMENTS  OF  GEOLOGY 

shorten  and  lengthen  their  courses,  withering  back  at  times  of 
drought  for  scores  of  miles,  or  even  for  a  hundred  miles  from 
the  point  reached  by  their  waters  during  seasons  of  rain. 

The  geological  work  of  streams.  The  work  of  streams  is  of 
three  kinds,  —  transportation,  erosion,  and  deposition.  Streams 
transport  the  waste  of  the  land  ;  they  wear,  or  erode,  their  chan- 
nels both  on  bed  and  banks;  and  they  deposit  portions  of  their 
load  from  time  to  time  along  their  courses,  finally  laying  it 
down  in  the  sea.  Most  of  the  work  of  streams  is  done  at  times 
of  flood. 

TRANSPORTATION 

The  invisible  load  of  streams.  Of  the  waste  which  a  river 
transports  we  may  consider  first  the  invisible  load  which  it  carries 
in  solution,  supplied  chiefly  by  springs  but  also  in  part  by  the 
run-off  and  from  the  solution  of  the  rocks  of  its  bed.  More 
than  half  the  dissolved  solids  in  the  water  of  the  average  river 
consists  of  the  carbonates  of  lime  and  magnesia ;  other  sub- 
stances are  gypsum,  sodium  sulphate  (Glauber's  salts),  mag- 
nesium sulphate  (Epsom  salts),  sodium  chloride  (common  salt), 
and  even  silica,  the  least  soluble  of  the  common  rock-making 
minerals.  The  amount  of  this  invisible  load  is  surprisingly 
large.  The  Mississippi,  for  example,  transports  each  year  113,- 
000,000  tons  of  dissolved  rock  to  the  Gulf. 

The  visible  load  of  streams.  This  consists  of  the  silt  which 
the  stream  carries  in  suspension,  and  the  sand  and  gravel  and 
larger  stones  which  it  pushes  along  its  bed.  Especially  in  times 
of  flood  one  may  note  the  muddy  water,  its  silt  being  kept  from 
settling  by  the  rolling,  eddying  currents ;  and  often  by  placing 
his  ear  close  to  the  bottom  of  a  boat  one  may  hear  the  clatter 
of  pebbles  as  they  are  hurried  along.  In  mountain  torrents  the 
rumble  of  bowlders  as  they  clash  together  may  be  heard  some 
distance  away.  The  amount  of  the  load  which  a  stream  can 
transport  depends  on  its  velocity.  A  current  of  two  thirds  of  a 


RIVERS  AND  A' ALLEYS  57 

mile  per  hour  can  move  fine  sand,  while  one  of  four  miles  per 
hour  sweeps  along  pebbles  as  large  as  hen's  eggs.  The  trans- 
porting power  of  a  stream  varies  as  the  sixth  power  of  its  velocity. 
If  its  velocity  is  multiplied  by  two,  its  transporting  power  is 
multiplied  by  the  sixth  power  of  two :  it  can  now  move  stones 
sixty-four  times  as  large  as  it  could  before. 

Stones  weigh  from  two  to  three  times  as  much  as  water,  and  in  water 
lose  the  weight  of  the  volume  of  water  which  they  displace.  What 
proportion,  then,  of  their  weight  in  air  do  stones  lose  when  submerged  ? 

Measurement  of  stream  loads.  To  obtain  the  total  amount  of 
waste  transported  by  a  river  is  an  important  but  difficult  matter. 
The  amount  of  water  discharged  must  first  be  found  by  multi- 
plying the  number  of  square  feet  in  the  average  cross  section  of 
the  stream  by  its  velocity  per  second,  giving  the  discharge  per 
second  in  cubic  feet.  The  amount  of  silt  to  a  cubic  foot  of 
water  is  found  by  filtering  samples  of  the  water  taken  from 
different  parts  of  the  stream  and  at  different  times  in  the  year, 
and  drying  and  weighing  the  residues.  The  average  amount  of 
silt  to  the  cubic  foot  of  water,  multiplied  by  the  number  of 
cubic  feet  of  water  discharged  per  year,  gives  the  total  load 
carried  in  suspension  during  that  time.  Adding  to  this  the 
estimated  amount  of  sand  and  gravel  rolled  along  the  bed, 
which  in  many  swift  rivers  greatly  exceeds  the  lighter  material 
held  in  suspension,  and  adding  also  the  total  amount  of  dis- 
solved solids,  we  reach  the  exceedingly  important  result  of  the 
total  load  of  waste  discharged  by  the  river.  Dividing  the 
volume  of  this  load  by  the  area  of  the  river  basin  gives 
another  result  of  the  greatest  geological  interest, —  the  rate  at 
which  the  region  is  being  lowered  by  the  combined  action  of 
weathering  and  erosion,  or  the  rate  of  denudation. 

The  rate  of  denudation  of  river  basins.  This  rate  varies  widely. 
The  Mississippi  basin  may  be  taken  as  a  representative  land 
surface  because  of  the  varieties  of  surface,  altitude  and  slope, 


58  THE  ELEMENTS  OF  GEOLOGY 

climate,  and  underlying  rocks  which  are  included  in  its  great 
extent.  Careful  measurements  show  that  the  Mississippi  basin 
is  now  being  lowered  at  a  rate  of  one  four-thousandth  of  a  foot 
a  year,  or  one  foot  in  four  thousand  years.  Taking  this  as  the 
average  rate  of  denudation  for  the  land  surfaces  of  the  globe, 
estimates  have  been  made  of  the  length  of  time  required  at  this 
rate  to  wash  and  wear  the  continents  to  the  level  of  the  sea. 
As  the  average  elevation  of  the  lands  of  the  globe  is  reckoned 
at  2411  feet,  this  result  would  occur  in  nine  or  ten  million 
years,  if  the  present  rate  of  denudation  should  remain  unchanged. 
But  even  if  no  movements  of  the  earth's  crust  should  lift  or 
depress  the  continents,  the  rate  of  wear  and  the  removal  of 
waste  from  their  surfaces  will  not  remain  the  same.  It  must 
constantly  decrease  as  the  lands  are  worn  nearer  to  sea  level 
and  their  slopes  become  more  gentle.  The  length  of  time 
required  to  wear  them  away  is  therefore  far  in  excess  of  that 
just  stated. 

The  drainage  area  of  the  Potomac  is  11,000  square  miles.  The  silt 
brought  down  in  suspension  in  a  year  would  cover  a  square  mile  to  the 
depth  of  four  feet.  At  what  rate  is  the  Potomac  basin  being  lowered 
from  this  cause  alone  ? 

It  is  estimated  that  the  Upper  Ganges  is  lowering  its  basin  at  the 
rate  of  one  foot  in  823  years,  and  the  Po  one  foot  in  720  years.  Why 
so  much  faster  than  the  Potomac  and  the  Mississippi? 

How  streams  get  their  loads.  The  load  of  streams  is  derived 
from  a  number  of  sources,  the  larger  part  being  supplied  by  the 
weathering  of  valley  slopes.  We  have  noticed  how  the  mantle 
of  waste  creeps  and  washes  to  the  stream  ways.  Watching  the 
run-off  during  a  rain,  as  it  hurries  muddy  with  waste  along  the 
gutter  or  washes  down  the  hillside,  we  may  see  the  beginning 
of  the  route  by  which  the  larger  part  of  their  load  is  delivered 
to  rivers.  Streams  also  secure  some  of  their  load  by  wearing  it 
from  their  beds  and  banks,  —  a  process  called  erosion. 


RIVERS  AND  VALLEYS  59 

EROSION 

Streams  erode  their  beds  chiefly  by  means  of  their  bottom 
load,  —  the  stones  of  various  sizes  and  the  sand  and  even  the  fine 
mud  which  they  sweep  along.  With  these  tools  they  smooth, 
grind,  and  rasp  the  rock  of  their  beds,  using  them  in  much  the 
fashion  of  sandpaper  or  a  file. 

Weathering  of  river  beds.  The  erosion  of  stream  beds  is 
greatly  helped  by  the  work  of  the  weather.  Especially  at  low 
water  more  or  less  of  the  bed  is  exposed  to  the  action  of  frost  and 
heat  and  cold,  joints 
are  opened,  rocks 
are  pried  loose  and 
broken  up  and  made 
ready  to  be  swept 
away  by  the  stream 
at  time  of  flood. 

Potholes.  In 
rapids  streams  also 
drill  out  their  rocky 
beds.  Where  some 

5lg  epressiOll  F](.   37    p0thole  in  Bed  of  Stream,  Ireland 

gives  rise  to  an 

eddy,  the  pebbles  which  gather  in  it  are  whirled  round  and 
round,  and,  acting  like  the  bit  of  an  auger,  bore  out  a  cylin- 
drical pit  called  a  pothole.  Potholes  sometimes  reach  a  depth 
of  a  score  of  feet.  Where  they  are  numerous  they  aid  mate- 
rially in  deepening  the  channel,  as  the  walls  between  them  are 
worn  away  and  they  coalesce. 

Waterfalls.  One  of  the  most  effective  means  of  erosion  which 
the  river  possesses  is  the  waterfall.  The  plunging  water  dis- 
lodges stones  from  the  face  of  the  ledge  over  which  it  pours, 
and  often  undermines  it  by  excavating  a  deep  pit  at  its  base. 
Slice  after  slice  is  thus  thrown  down  from  the  front  of  the 


60 


THE  ELEMENTS   OF   GEOLOGY 


FIG.  38.   Map  of  the  Gorge  of  the 
Niagara  River 


cliff,  and  the  cataract  cuts 
its  way  upstream  leaving  a 
gorge  behind  it. 

Niagara  Falls.  The  Niag- 
ara River  flows  from  Lake 
Erie  at  Buffalo  in  a  broad 
channel  which  it  has  cut  but 
a  few  feet  below  the  level  of 
the  region.  Some  thirteen 
miles  from  the  outlet  it 
plunges  over  a  ledge  one 
hundred  and  seventy  feet 
high  into  the  head  of  a  nar- 
row gorge  which  extends  for 
seven  miles  to  the  escarp- 
ment of  the  upland  in  which 
the  gorge  is  cut.  The  strata 
which  compose  the  upland 
dip  gently  upstream  and  con-' 
sist  at  top  of  a  massive  lime- 
stone, at  the  Falls  about 
eighty  feet  thick,  and  below 
of  soft  and  easily  weathered 
shale.  Beneath  the  Falls  the 
underlying  shale  is  cut  and 
washed  away  by  the  descend- 
ing water  and  retreats  also 
because  of  weathering,  while 
the  overhanging  limestone 
breaks  down  in  huge  blocks 
from  time  to  time. 


Niagara  is  divided  by  Goat  Island  into  the  Horseshoe  Falls 
and  the  American  Falls.  The  former  is  supplied  by  the  main 
current  of  the  river,  and  from  the  semicircular  sweep  of  its 


RIVERS  AND  VALLEYS  61 

rim  a  sheet  of  water  in  places  at  least  fifteen  or  twenty  feet 
deep  plunges  into  a  pool  a  little  less  than  two  hundred  feet  in 
depth.  Here  the  force  of  the  falling  water  is  sufficient  to  move 
about  the  fallen  blocks  of  limestone  and  use  them  in  the  exca- 
vation of  the  shale  of  the  bed.  At  the  American  Falls  the 
lesser  branch  of  the  river,  which  flows  along  the  American 
side  of  Goat  Island,  pours  over  the  side  of  the  gorge  and  breaks 
upon  a  high  talus  of  limestone  blocks  which  its  smaller  volume 
of  water  is  unable  to  grind  to  pieces  and  remove. 

A  series  of  surveys  have  determined  that  from  1842  to  1911 
the  Horseshoe  Falls  retreated  at  the  rate  of  about  five  feet  a  year, 
while  the  American  Falls  retreated  at  about  one  twentieth  of 


R       W  EN 


012345 

FIG.  39.  Longitudinal  Section  of  Niagara  Gorge 

Black,  water;  F,  falls;  R,  rapids;  W,  whirlpool;  E,  escarpment; 
N,  north ;  S,  south 

this  rate.  We  cannot  doubt  that  the  same  agency  which  is 
now  lengthening  the  gorge  at  this  rapid  rate  has  cut  it  back 
its  entire  length  of  seven  miles. 

While  Niagara  Falls  have  been  cutting  back  a  gorge  seven 
miles  long  and  from  two  hundred  to  three  hundred  feet  deep, 
the  river  above  the  Falls  has  eroded  its  bed  scarcely  below  the 
level  of  the  upland  on  which  it  flows.  Like  all  streams  which 
are  the  outlets  of  lakes,  the  Niagara  flows  out  of  Lake  Erie  clear 
of  sediment,  as  from  a  settling  basin,  and  carries  no  tools  with 
which  to  abrade  its  bed.  We  may  infer  from  this  instance  how 
slight  is  the  erosive  power  of  clear  water  on  hard  rock. 

Assuming  that  the  rate  of  recession  of  the  combined  volumes  of  the 
American  and  Horseshoe  Falls  was  five  feet  a  year  below  Goat  Island 
and  assuming  that  this  rate  has  been  uniform  in  the  past,  how  long  is  it 
since  the  Niagara  River  fell  over  the  edge  of  the  escarpment  where 
now  is  the  mouth  of  the  present  gorge? 


62  THE  ELEMENTS  OF  GEOLOGY 

The  profile  of  the  bed  of  the  Niagara  along  the  gorge  (Fig.  39)  shows 
alternating  deeps  and  shallows  which  cannot  be  accounted  for,  except 
in  a  single  instance,  by  the  relative  hardness  of  the  rocks  of  the  river 
bed.  The  deeps  do  not  exceed  that  at  the  foot  of  the  Horseshoe  Falls 
at  the  present  time.  When  the  gorge  was  being  cut  along  the  shallows, 
how  did  the  Falls  compare  in  excavating  power,  in  force,  and  volume 
with  the  Niagara  of  to-day?  How  did  the  rate  of  recession  at  those 
times  compare  with  the  present  rate  ?  Is  the  assumption  made  above 
that  the  rate  of  recession  has  been  uniform  correct? 

The  first  stretch  of  shallows  below  the  Falls  causes  a  tumultuous 
rapid  impossible  to  sound.  Its  depth  has  been  estimated  at  thirty-five 
feet.  From  what  data  could  such  an  estimate  be  made  ? 

Suggest  a  reason  why  the  Horseshoe  Falls  are  convex  upstream. 

At  the  present  rate  of  recession  which  will  reach  the  head  of  Goat 
Island  the  sooner,  the  American  or  the  Horseshoe  Falls?  What  will 
be  the  fate  of  the  Falls  left  behind  when  the  other  has  passed  beyond 
the  head  of  the  island  ? 

The  rate  at  Avhich  a  stream  erodes  its  bed  depends  in  part  upon  the 
nature  of  the  rocks  over  which  it  flows.  Will  a  stream  deepen  its  chan- 
nel more  rapidly  on  massive  or  on  thin-bedded  and  close-jointed  rocks? 
on  horizontal  strata  or  on  strata  steeply  inclined  ? 

DEPOSITION 

While  the  river  carries  its  invisible  load  of  dissolved  rock  on 
without  stop  to  the  sea,  its  load  of  visible  waste  is  subject  to 
many  delays  en  route.  Now  and  again  it  is  laid  aside,  to  be 
picked  up  later  and  carried  some  distance  farther  on  its  way. 
One  of  the  most  striking  features  of  the  river  therefore  is  the 
waste  accumulated  along  its  course,  in  bars  and  islands  in  the 
channel,  beneath  its  bed,  and  in  flood  plains  along  its  banks. 
All  this  alluvium,  to  use  a  general  term  for  river  deposits, 
with  which  the  valley  is  cumbered  is  really  en  route  to  the  sea ; 
it  is  only  temporarily  laid  aside  to  resume  its  journey  later  on. 
Constantly  the  river  is  destroying  and  rebuilding  its  alluvial 
deposits,  here  cutting  and  there  depositing  along  its  banks, 
here  eroding  and  there  building  a  bar,  here  excavating  its  bed 


64 


THE  ELEMENTS  OF  GEOLOGY 


and  there  filling  it  up,  and  at  all  times  carrying  the  material 
picked  up  at  one  point  some  distance  on  downstream  before 
depositing  it  at  another. 

These  deposits  are  laid  down  by  slackening  currents  where 
the  velocity  of  the  stream  is  checked,  as  011  the  inner  side  of 
curves,  and  where  the  slope  of  the  bed  is  diminished,  and  in  the 
lee  of  islands,  bridge  piers  and  projecting  points  of  land.  How 
slight  is  the  check  required  to  cause  a  current  to  drop  a  large 


FIG.  41.   Sand  Bar  deposited  by  Stream,  showing  Cross  Bedding 

part  of  its  load  may  be  inferred  from  the  law  of  the  relation 
of  the  transporting  power  to  the  velocity.  If  the  velocity  is 
decreased  one  half,  the  current  can  move  fragments  but  one 
sixty-fourth  the  size  of  those  which  it  could  move  before,  and 
must  drop  all  those  of  larger  size. 

Will  a  river  deposit  more  at  low  water  or  at  flood  ?   when  rising  or 
when  falling? 

Stratification.    Paver  deposits  are  stratified,  as  may  be  seen  in 
any  fresh  cut  in  banks  or  bars.    The  waste  of  which  they  are 


RIVERS  AND  VALLEYS  65 

luiilt  has  been  sorted  and  deposited  in  layers,  one  above  another ; 
some  of  finer  and  some  of  coarser  material.  The  sorting  action 
of  running  water  depends  on  the  fact  that  its  transporting 
power  varies  with  the  velocity.  A  current  whose  diminishing 
velocity  compels  it  to  drop  coarse  gravel,  for  example,  is  still 
able  to  move  all  the  finer  waste  of  its  load,  and  separating 
it  from  the  gravel,  carries  it  on  downstream;  while  at  a  later 
time  slower  currents  may  deposit  on  the  gravel  bed  layers  of 
sand,  and,  still  later,  slack  water  may  leave  on  these  a  layer  of 
mud.  In  case  of  materials  lighter  than  water  the  transporting 
power  does  not  depend  on  the  velocity,  and  logs  of  wood,  for 
instance,  are  floated  on  to  the  sea  on  the  slowest  as  well  as  on 
the  most  rapid  currents. 

Cross  bedding.  A  section  of  a  bar  exposed  at  low  water  may 
show  that  it  is  formed  of  layers  of  sand,  or  coarser  stuff,  inclined 
downstream  as  steeply  often  as  the  angle  of  repose  of  the 
material.  From  a  M — r _'...— — - — 

CL      •• '  ••  /, 

boat  anchored  over      — "irvr"' "^  — — ^ 

the  lower  end  of  a         FlG'  42'  Longitudinal  Section  of  a  River  Bar 

submerged  sand  bar  we  may  observe  the  way  in  which  this 
structure,  called  cross  bedding,  is  produced.  Sand  is  continually 
pushed  over  the  edge  of  the  bar  at  b  (Fig.  42)  and  comes  to  rest 
in  successive  layers  on  the  sloping  surface.  At  the  same  time 
the  bar  may  be  worn  away  at  the  upper  end,  a,  and  thus  slowly 
advance  down  stream.  While  the  deposit  is  thus  cross  bedded, 
it  constitutes  as  a  whole  a  stratum  whose  upper  and  lower 
surfaces  are  about  horizontal.  In  sections  of  river  banks  one 
may  often  see  a  vertical  succession  of  cross-bedded  strata,  each 
built  in  the  way  described. 

Water  wear.  The  coarser  material  of  river  deposits,  such  as 
cobblestones,  gravel,  and  the  larger  grains  of  sand,  are  water  worn, 
or  rounded,  except  when  near  their  source.  Rolling  along  the 
bottom  they  have  been  worn  round  by  impact  and  friction  as 
they  rubbed  against  one  another  and  the  rocky  bed  of  the  stream. 


66 


THE  ELEMENTS  OF  GEOLOGY 


Experiments  have  shown  that  angular  fragments  of  granite  lose 
nearly  half  their  weight  and  become  well  rounded  after  traveling  fif- 
teen miles  in  rotating  cylinders  partly  filled  with  water.  Marbles 
are  cheaply  made  in  Germany  out  of  small  limestone  cubes  set  revolving 


FIG.  43.   Water-Worn  Pebbles,  Upper  Potomac  River,  Maryland 

in  a  current  of  water  between  a  rotating  bed  of  stone  and  a  block 
of  oak,  the  process  requiring  but  about  fifteen  minutes.  It  has  been 
found  that  in  the  upper  reaches  of  mountain  streams  a  descent  of  less 
than  a  mile  is  sufficient  to  round  pebbles  of  granite. 


LAND  FORMS  DUE  TO  EIVER  EROSION 

River  valleys.  In  their  courses  to  the  sea,  rivers  follow  val- 
leys of  various  forms,  some  shallow  and  some  deep,  some 
narrow  and  some  wide.  Since  rivers  are  known  to  erode  their 
beds  and  banks,  it  is  a  fair  presumption  that,  aided  by  the 
weather,  they  have  excavated  the  valleys  in  which. they  now. 

Moreover,  a  bird's-eye  view  or  a  map  of  a  region  shows  the 
significant  fact  that  the  valleys  of  a  system  unite  with  one 
another  in  a  branch  work,  as  twigs  meet  their  stems  and  the 


RIVERS  AND  VALLEYS  67 

branches  of  a  tree  its  trunk.  Each  valley,  from  that  of  the 
smallest  rivulet  to  that  of  the  master  stream,  is  proportionate 
to  the  size  of  the  stream  which  occupies  it.  With  a  few 
explainable  exceptions  the  valleys  of  tributaries  join  that  of 
the  trunk  stream  at  a  level ;  there  is  no  sudden  descent  or 
break  in  the  bed  at  the  point  of  juncture.  These  are  the 
natural  consequences  which  must  follow  if  the  land  has  long 
been  worked  upon  by  streams,  and  no  other  process  has  ever 
been  suggested  which  is  competent  to  produce  them.  We  must 
conclude  that  valley  systems -have  been  formed  by  the  river 
systems  which  drain  them,  aided  by  the  work  of  the  weather ; 
they  are  not  gaping  fissures  in  the  earth's  crust,  as  early  ob- 
servers imagined,  but  are  the  furrows  which  running  water  has 
drawn  upon  the  land. 

As  valleys  are  made  by  the  slow  wear  of  streams  and  the 
action  of  the  weather,  they  pass  in  their  development  through 
successive  stages,  each  of  which  has  its  own  characteristic 
features.  We  may  therefore  classify  rivers  and  valleys  accord- 
ing to  the  stage  which  they  have  reached  in  their  life  history 
from  infancy  to  old  age. 

Young  River  Valleys 

Infancy.  The  Red  River  of  the  North.  A  region  in  northwestern 
Minnesota  and  the  adjacent  portions  of  North  Dakota  and  Manitoba  was 
so  recently  covered  by  the  waters  of  an  extinct  lake,  known  as  Lake 
Agassiz,  that  the  surface  remains  much  as  it  was  left  when  the  lake 
was  drained  away.  The  flat  floor,  spread  smooth  with  lake-laid  silts,  is 
still  a  plain,  to  the  eye  as  level  as  the  sea.  Across  it  the  Red  River  of 
the  North  and  its  branches  run  in  narrow,  ditch-like  channels,  steep- 
sided  and  shallow,  not  exceeding  sixty  feet  in  depth,  their  gradients 
differing  little  from  the  general  slopes  of  the  region.  The  trunk  streams 
have  but  few  tributaries  ;  the  river  system,  like  a  sapling  with  few 
limbs,  is  still  undeveloped.  Along  the  banks  of  the  trunk  streams  short 
gullies  are  slowly  lengthening  headwards,  like  growing  twigs  which 
are  sometime  to  become  large  branches. 


68 


RIVERS  AND  VALLEYS  69 

The  flat  interstream  areas  are  as  yet  but  little  scored  by  drainage 
lines,  and  in  wet  weather  water  lingers  in  ponds  in  any  initial  depres- 
sions on  the  plain. 

Contours.  In  order  to  read  the  topographic  maps  of  the  text-book  and 
the  laboratory  the  student  should  know  that  contours  are  lines  drawn 
on  maps  to  represent  relief,  all  points  on  any  given  contour  being  of  equal 
height  above  sea  level.  The  contour  interval  is  the  uniform  vertical 
distance  between  two  adjacent  contours  and  varies  on  different  maps. 


FIG.  45.  A  Young  River,  Iowa 

Note  that  it  has  hardly  begun  to  cut  a  valley  in  the  plain  of  glacial 
drift  on  which  it  flows 

To  express  regions  of  faint  relief  a  contour  interval  of  ten  or  twenty 
feet  is  commonly  selected;  while  in  mountainous  regions  a  contour 
interval  of  two  hundred  and  fifty,  five  hundred,  or  even  one  thousand 
feet  may  be  necessary  in  order  that  the  contours  may  not  be  too 
crowded  for  easy  reading. 

Whether  a  river  begins  its  life  on  a  lake  plain,  as  in  the 
example  just  cited,  or  upon  a  coastal  plain  lifted  from  beneath 
the  sea  or  on  a  spread  of  glacial  drift  left  by  the  retreat  of 
continental  ice  sheets,  such  as  covers  much  of  Canada  and  the 
northeastern  parts  of  the  United  States,  its  infantile  stage  pre- 
sents the  same  characteristic  features,  —  a  narrow  and  shallow 
valley,  with  undeveloped  tributaries  and  undrained  interstream 
areas.  Ground  water  stands  high,  and,  exuding  in  the  undrained 
initial  depressions,  forms  marshes  and  lakes. 


70 


THE  ELEMENTS  OF   GEOLOGY 


Lakes.  Lakes  are  perhaps  the  most  obvious  of  these  fleeting 
features  of  infancy.  They  are  short-lived,  for  their  destruction 
is  soon  accomplished  by  several  means.  As  a  river  system  ad- 
vances toward  maturity  the  deepening  and  extending  valleys  of 
the  tributaries  lower  the  ground-water  surface  and  invade  the 
undrained  depressions  of  the  region.  Lakes  having  outlets  are 


FIG.  46.    A  Young  Drift  Region  in  Wisconsin 

Describe  this  area.  How  high  are  the  hills?  Are  they  such  in  form  and 
position  as  would  be  left  by  stream  erosion  ?  Consult  a  map  of  the  entire 
state  and  notice  that  the  Fox  River  finds  way  to  Lake  Michigan,  while 
the  Wisconsin  empties  into  the  Mississippi.  Describe  that  portion  of  the 
divide  here  shown  between  the  Mississippi  and  the  St.  Lawrence  systems. 
Which  is  the  larger  river,  the  Wisconsin  or  the  Fox  ?  Other  things  being 
equal,  which  may  be  expected  to  deepen  its  bed  the  more  rapidly  ? 
What  changes  are  likely  to  occur  when  one  of  these  rivers  comes  to  flow 
at  a  lower  level  than  the  other?  Why  have  not  these  changes  occurred 
already  ? 

drained  away  as  their  basin  rims  are  cut  down  by  the  outflow- 
ing streams,  —  a  slow  process  where  the  rim  is  of  hard  rock, 
but  a  rapid  one  where  it  is  of  soft  material  such  as  glacial  drift. 
Lakes  are  effaced  also  by  the  filling  of  their  basins.  Inflow- 
ing streams  and  the  wash  of  rains  bring  in  waste.  Waves  abrade 
the  shore  and  strew  the  debris  worn  from  it  over  the  lake  bed. 
Shallow  lakes  are  often  filled  with  organic  matter  from  decay- 
ing vegetation. 

Does  the  outflowing  stream  from  a  lake  carry  sediment?    How  does 
this  fact  affect  its  erosive  power  on  hard  rock  ?    on  loose  material  ? 


RIVERS  AND  VALLEYS 


71 


Lake  Geneva  is  a  well-known  example  of  a  lake  in  process  of  obliter- 
ation. The  inflowing  Rhone  has  already  displaced  the  waters  of  the 
lake  for  a  length  of  twenty  miles  with  the  waste  brought  down  from 
the  high  Alps.  For  this  distance  there  extends  up  the  Rhone  Valley 
an  alluvial  plain,  which  has  grown  lakeward  at  the  rate  of  a  mile  and 
a  half  since  Roman  times,  as  proved  by  the  distance  inland  at  which 
a  Roman  port  now  stands. 

How  rapidly  a  lake  may  be  silted  up  under  exceptionally  favorable 
conditions  is  illustrated  by  the  fact  that  over  the  bottom  of  the  artificial 
lake,  of  thirty-five  square  miles,  formed 
behind  the  great  dam  across  the  Colorado 
River  at  Austin,  Texas,  sediments  thirty- 
nine  feet  deep  gathered  in  seven  years. 

Lake  Mendota,  one  of  the  many  beauti- 
ful  lakes  of  southern  Wisconsin,  is  rapidly 
cutting  back  the  soft  glacial  drift  of  its 
shores  by  means  of  the  abrasion  of  its 
waves.  While  the  shallow  basin  is  thus 
broadened,  it  is  also  being  filled  with  the 
waste  ;  and  the  time  is  brought  nearer  when  it  will  be  so  shoaled  that 
vegetation  can  complete  the  work  of  its  effacement. 


A  Small  Lake  being 
broadened  and  shoaled  by 
Wave  Wear 

Is,  lake  surface;  dotted  line, 
initial  shore ;  a,  cut  made  by 
waves;  6,  fill  made  of  mate- 
rial taken  from  a 


FIG.  48.    A  Lake  in  Process  of  Effacement,  Montana 
By  what  means  is  the  lake  bed  being  filled  ? 


72  THE  ELEMENTS  OF  GEOLOGY 

Along  the  margin  of  a  shallow  lake  mosses,  water  lilies, 
grasses,  and  other  water-loving  plants  grow  luxuriantly.  As 
their  decaying  remains  accumulate  on  the  bottom,  the  ring  of 
marsh  broadens  inwards,  the  lake  narrows  gradually  to  a  small 
pond  set  in  the  midst  of  a  wide  bog,  and  finally  disappears. 
All  stages  in  this  process  of  extinction  may  be  seen  among 
the  countless  lakelets  which  occupy  sags  in  the  recent  sheets 


FIG.  49.   A  Level  Meadow,  Scotland 
Explain  its  origin.    What  will  be  its  future? 

of  glacial  drift  in  the  northern  states ;  and  more  numerous  than 
the  lakes  which  still  remain  are  those  already  thus  filled  with 
carbonaceous  matter  derived  from  the  carbon  dioxide  of  the 
atmosphere.  Such  fossil  lakes  are  marked  by  swamps  or  level 
meadows  underlain  with  muck. 

The  advance  to  maturity.  The  infantile  stage  is  brief.  As 
a  river  advances  toward  maturity  the  initial  depressions,  the 
lake  basins  of  its  area,  are  gradually  effaced.  By  the  furrowing 
action  of  the  rain  wash  and  the  headward  lengthening  of  tribu- 
taries a  branchwork  of  drainage  channels  grows  until  it  covers 
the  entire  area,  and  not  an  acre  is  left  on  which  the  fallen 


RIVERS  AND  VALLEYS 


73 


raindrop  does  not  find  already  cut  for  it  an  uninterrupted  down- 
ward path  which  leads  it  on  by  way  of  gully,  brook,  and  river 
to  the  sea.  The  initial  surface  of  the  land,  by  whatever  agency 


FIG.  50.   Drainage  Maps 

A,  an  area  in  its  infancy,  Buena  Vista  County,  Iowa;  B,  an  area  in  its 
maturity,  Riuggold  County,  Iowa 

it  was  modeled,  is  now  wholly  destroyed ;  the  region  is  all 
reduced  to  valley  slopes. 

The  longitudinal  profile  of  a  stream.  This  at  first  corresponds 
with  the  initial  surface  of  the  region  on  which  the  stream 
begins  to  flow,  although  its  way  may  lead  through  basins  and 
down  steep  descents. 

The  successive  pro-    ^  \   v 

files  to  which  it  re- 
duces its  bed  are 
illustrated  in  Fig- 
ure 51.  As  the  gra- 
dient, or  rate  of  de- 
scent of  its  bed,  is 
lowered,  the  veloc- 
ity of  the  river  is 
decreased  until  its 


FIG.  51.  Successive  Longitudinal  Profiles 
of  a  Stream 

am,  initial  profile,  with  waterfall  at  w,  and  basins  at  I 
and  I',  which  at  first  are  occupied  by  lakes  and 
later  are  filled  or  drained ;  6,  c,  d,  and  e,  profiles 
established  in  succession  as  the  stream  advances 
from  infancy  toward  old  age.  Note  that  these 
profiles  are  concave  toward  the  sky.  This  is  the 
erosion  curve.  What  contrasting  form  has  the 
weather  curve  (p.  34)  ? 


74  THE  ELEMENTS  OF  GEOLOGY 

lessening  energy  is  wholly  consumed  in  carrying  its  load  and  it 
can  no  longer  erode  its  bed.  The  river  is  now  at  grade,  and  its 
capacity  is  just  equal  to  its  load.  If  now  its  load  is  increased 
the  stream  deposits,  and  thus  builds  up,  or  aggrades,  its  bed. 
On  the  other  hand,  if  its  load  is  diminished  it  has  energy  to 
spare,  and  resuming  its  w^ork  of  erosion,  degrades  its  bed.  In 
either  case  the  stream  continues  aggrading  or  degrading  until 


FIG.  52.   A  V-Valley,  —  the  Canyon  of  the  Yellowstone 

Note  the  steep  sides.  What  processes  are  at  work  upon  them  ?  How  wide 
is  the  valley  at  base  compared  with  the  width  of  the  stream  ?  Do  you 
see  any  river  deposits  along  its  banks?  Is  the  stream  flowing  swiftly 
over  a  rock  bed,  or  quietly  over  a  bed  which  it  has  built  up?  Is  it 
graded  or  ungraded?  Note  that  the  canyon  walls  project  in  interlock- 
ing spurs 

a  new  gradient  is  found  where  the  velocity  is  just  sufficient  to 
move  the  load,  and  here  again  it  reaches  grade. 

V-Valleys.  Vigorous  rivers  well  armed  with  waste  make  short 
work  of  cutting  their  beds  to  grade,  and  thus  erode  narrow, 
steep-sided  gorges  only  wide  enough  at  the  base  to  accommodate 
the  stream.  The  steepness  of  the  valley  slopes  depends  on  the 
relative  rates  at  which  the  bed  is  cut  down  by  the  stream  and 
the  sides  are  worn  back  by  the  weather.  In  resistant  rock  a 


RIVERS  AND  VALLEYS 


75 


swift,  well-laden  stream  may  saw  out  a  gorge  whose  sides  are 
nearly  or  even  quite  vertical,  but  as  a  rule  young  valleys 
whose  streams  have  not  yet  reached  grade  are  V-shaped ;  their 
sides  flare  at  the  top  because  here  the  rocks  have  longest  been 
opened  up  to  the  action  of  the  weather.  Some  of  the  deepest 
canyons  may  be  found  where  a 
rising  land  mass,  either  mountain 
range  or  plateau,  has  long  main- 
tained by  its  continued  uplift  the 
rivers  of  the  region  above  grade. 

In  the  northern  hemisphere  the  north  FlG- 
sides  of  river  valleys  are  sometimes  of 


500     1000     1500 


FEET 


Neglecting  any  cutting  of  the 
river  against  its  banks,  estimate 
what  part  of  the  excavation  of 
the  canyon  is  due  to  the  vertical 
erosion  of  its  bed  by  the  river 
and  what  to  weathering  and 
rain  wash  on  the  canyon  sides 


Section  of  the  Yellow- 
more  gentle  slope  than  the  south  sides.  This  canvon  is  100°  feet  deeP'  2500 
0  feet  wide  at  the  top,  and  about 

Can  you  suggest  a  reason?  m  feet  wide  at  tbe  bottom 

The  Grand  Canyon  of  the  Colorado 
River  in  Arizona.  The  Colorado  River 
trenches  the  high  plateau  of  northern 
Arizona  with  a  colossal  canyon  two 
hundred  and  eighteen  miles  long  and 
more  than  a  mile  in  greatest  depth 
(Fig.  15).  The  rocks  in  which  the  canyon  is  cut  are  for  the  most  part 
flat-lying,  massive  beds  of  limestones  and  sandstones,  with  some  shales, 
beneath  which  in  places  harder  crystalline  rocks  are  disclosed.  Where 
the  canyon  is  deepest  its  walls  have  been  profoundly  dissected.  Lateral 
ravines  have  widened  into  immense  amphitheaters,  leaving  between 
them  long  ridges  of  mountain  height,  buttressed  and  rebuttressed  with 
flanking  spurs  and  carved  into  majestic  architectural  forms.  From  the 
extremity  of  one  of  these  promontories  it  is  two  miles  or  more  across 
the  gulf  to  the  point  of  the  one  opposite,  and  the  heads  of  the  amphi- 
theaters are  thirteen  miles  apart. 

The  lower  portion  of  the  canyon  is  much  narrower  (Fig.  54)  and  its 
svalls  of  dark  crystalline  rock  sink  steeply  to  the  edge  of  the  river,  a 
swift,  powerful  stream  a  few  hundred  feet  wide,  turbid  with  reddish 
silt,  by  means  of  which  it  continually  rasps  its  rocky  bed  as  it  hurries 
on.  The  Colorado  is  still  deepening  its  gorge.  In  the  Grand  Canyon 
its  gradient  is  seven  and  one  half  feet  to  the  mile,  but,  as  in  all 
ungraded  rivers,  the  descent  is  far  from  uniform.  Graded  reaches  in 


76 


RIVERS  AND  VALLEYS  77 

soft  rock  alternate  with  steeper  declivities  in  hard  rock,  forming  rapids 
such  as,  for  example,  a  stretch  of  ten  miles  where  the  fall  averages 
twenty-one  feet  to  the  mile.  Because  of  these  dangerous  rapids  the  few 
exploring  parties  who  have  traversed  the  Colorado  canyon  have  done  so 
at  the  hazard  of  their  lives. 

The  canyon  has  been  shaped  by  several  agencies.  Its  depth  is  due 
to  the  river  which  has  sawed  its  way  far  toward  the  base  of  a  lofty 
rising  plateau.  Acting  alone  this  would  have  produced  a  slitlike  gorge 
little  wider  than  the  breadth  of  the  stream.  The  impressive  width  of 
the  canyon  and  the  magnificent  architectural  masses  which  fill  it  are 
owing  to  two  causes.  Running  water  has  gulched  the  walls  and 
weathering  has  everywhere  attacked  and  driven  them  back.  The  hori- 
zontal harder  beds  stand  out  in  long  lines  of  vertical  cliffs,  often  hun- 
dreds of  feet  in  height,  at  whose  feet  talus  slopes  conceal  the  outcrop 


B 
FIG.  66.  Diagrams  illustrating  Conditions  which  produce  Falls  or  Rapids 

A,  vertical  succession  of  harder  and  softer  rocks ;  B,  horizontal  succession  of  the 
same.  In  A  the  stream  ab  in  sinking  its  bed  through  a  mass  of  strata  of  dif- 
ferent degrees  of  hardness  has  discovered  the  weak  layer  s  beneath  the  hard 
layer  h.  It  rapidly  cuts  its  way  in  *,  while  in  h  its  work  is  delayed.  Thus 
the  profile  a/6'  is  soon  reached,  with  falls  at/.  In  B  the  initial  profile  is 
shown  by  dotted  line. 

of  the  weaker  strata  (Fig.  15).  As  the  upper  cliffs  have  been  sapped 
and  driven  back  by  the  weather,  broad  platforms  are  left  at  their  bases 
and  the  sides  of  the  canyon  descend  to  the  river  by  gigantic  steps.  Far 
up  and  down  the  canyon  the  eye  traces  these  horizontal  layers,  like  the 
flutings  of  an  elaborate  molding,  distinguishing  each  by  its  contour  as 
well  as  by  its  color  and  thickness. 

The  Grand  Canyon  of  the  Colorado  is  often  and  rightly  cited  as  an 
example  of  the  stupendous  erosion  which  may  be  accomplished  by  a 
river.  And  yet  the  Colorado  is  a  young  stream  and  its  work  is  no  more 
than  well  begun.  It  has  not  yet  wholly  reached  grade,  and  the  great 
task  of  the  river  and  its  tributaries — the  task  of  leveling  the  lofty 
plateau  to  a  low  plain  and  of  transporting  it  grain  by  grain  to  the  sea  — 
still  lies  almost  entirely  in  the  future. 


78 


THE  ELEMENTS  OF  GEOLOGY 


lv  tnf 


!c 


Waterfalls  and  rapids.  Before  the  bed  of  a  stream  is  reduced 
to  grade  it  may  be  broken  by  abrupt  descents  which  give  rise  to 
waterfalls  and  rapids.  Such  breaks  in  a  river's  bed  may  belong 

to  the  initial  surface  over  which 
it  began  its  course ;  still  more 
commonly  are  they  developed 
in  the  rock  mass  through  which 
it  is  cutting  its  valley.  Thus, 
wherever  a  stream  leaves  harder 
rocks  to  flow  over  softer  ones  the 

to,  lavas  deeply  decayed  through    latter  are  quickly  worn  below  the 
action  of  thermal  waters ;  m  and    level  of  the  former,  and  a  sharp 

m',  masses  of  undecayed  lavas  to  i                      -\               -.T                     p  -n 

whose  hardness  the  falls  are  dne.  diange  m  sl°Pe>  Wlth  a  Waterfall 

Which  fall  will  be  worn  away  the  or  rap^d,  results  (Fig.  55). 
sooner?    How  far  upstream  will 


FIG.  56.  Longitudinal  Section  of 
Yellowstone  River  at  Lower 
Fall,  F,  and  Upper  Fall,  F', 
Yellowstone  National  Park 


each  fall  migrate?  Draw  profile 
of  the  river  when  one  fall  has  dis- 
appeared 


At  time  of  flood  young  tributaries 
with-  steeper  courses  than  that  of  the 
trunk  stream  may  bring  down  stone? 

and  finer  waste,  which  the  gentler  current  cannot  move  along,  ana 
throw  them  as  a  dam  across  its  way.  The  rapids  thus  formed  are  also 
ephemeral,  for  as  the  gradient 
of  the  tributaries  is  lowered  the 
main  stream  becomes  able  to 
handle  the  smaller  and  finer 
load  which  they  discharge. 

A  rare  class  of  falls  is  pro- 
duced where  the  minor  tribu- 
taries of  a  young  river  are  not 
able  to  keep  pace  with  their 
master  stream  in  the  erosion 
of  their  beds  because  of  their 
smaller  volume,  and  thus  join 
it  by  plunging  over  the  side 


FIG.  57.  Diagram  illustrating  Migration 
of  a  Fall  due  to  a  Hard  Layer  77,  in 
the  Midst  of  Soft  Layers  S  and  S,  all 
dipping  upstream 

a,  bt  c,  d,  and  e,  successive  profiles  of  the 
stream ;  /,  /',  and  /",  successive  posi- 
tions of  the  fall ;  r,  rapid  to  which  the 
fall  is  reduced.  Draw  diagram  showing 
migration  of  fall  in  strata  dipping  down- 
stream. Under  what  conditions  of  incli- 
nation of  the  strata  will  a  fall  migrate 
the  farthest  and  have  the  longest  life? 
Under  what  conditions  will  it  migrate  the 
least  distance  and  soonest  be  destroyed  ? 


of  its  gorge.    But  as  the  river 

approaches  grade  and  slackens 

its  down  cutting,  the   tributaries   sooner   or   later   overtake   it,   and, 

effacing  their  falls,  unite  with  it  on  a  level. 


Contour  Interval  100  feet 


FIG.  58.    Maturely  Dissected  Plateau  near  Charleston,  West  Virginia 

Compare  the  number  of  streams  in  any  given  number  of  square  miles 
with  the  number  on  an  area  of  the  same  size  in  the  Red  River  valley 
(Fig.  44).  What  is  the  shape  of  the  ridges?  Are  their  summits  broad 
or  narrow  ?  Are  their  crests  even  or  broken  by  knobs  and  cols  (the 
depressions  on  the  crest  line)  ?  If  the  latter,  how  deeply  have  the  cols 
been  worn  beneath  the  summits  of  the  knobs  ? 
79 


80  THE  ELEMENTS  OF   GEOLOGY 

Waterfalls  and  rapids  of  all  kinds  are  evanescent  features  of 
a  river's  youth.  Like  lakes  they  are  soon  destroyed,  and  if  any 
long  time  had  already  elapsed  since  their  formation  they  would 
have  been  obliterated  already. 

Local  baselevels.  That  balanced  condition  called  grade,  where 
a  river  neither  degrades  its  bed  by  erosion  nor  aggrades  it  by 
deposition,  is  first  attained  along  reaches  of  soft  rocks,  ungraded 
outcrops  of  hard  rocks  remaining  as  barriers  which  give  rise  to 
rapids  or  falls.  Until  these  barriers  are  worn  away  they  con- 
stitute local  baselevels,  below  which  level  the  stream,  up  valley 


FIG.  59.   A  Maturely  Dissected  Region  of  Slight  Relief,  Iowa 

from  them,  cannot  cut.  They  are  eroded  to  grade  one  after 
another,  beginning  with  the  least  strong,  or  the  one  nearest 
the  mouth  of  the  stream.  In  a  similar  way  the  surface  of  a 
lake  in  a  river's  course  constitutes  for  all  inflowing  streams 
a  local  baselevel,  which  disappears  when  the  basin  is  filled  or 
drained. 

Mature  and   Old  Rivers 

Maturity  is  the  stage  of  a  river's  complete  development  and 
most  effective  work.  The  river  system  now  has  well  under  way 
its  great  task  of  wearing  down  the  land  mass  which  it  drains 
and  carrying  it  particle  by  particle  to  the  sea.  The  relief  of  the 
land  is  now  at  its  greatest ;  for  the  main  channels  have  been 


RIVERS  AND  VALLEYS 


81 


sunk  to  grade,  while 
the  divides  remain 
but  little  worn  below 
their  initial  altitudes. 
Ground  water  now 
stands  low.  The  run- 
off washes  directly  to 
the  streams,  writh  the 
least  delay  and  loss 
by  evaporation  in 
ponds  and  marshes; 
the  discharge  of  the 
river  is  therefore  at 
its  height.  The  entire 
region  is  dissected  by 
stream  ways.  The 
area  of  valley  slopes 
is  now  largest  and 
sheds  to  the  streams 
a  heavier  load  of 
waste  than  ever  be- 
fore. At  maturity  the 
river  system  is  doing 
its  greatest  amount  of 
work  both  in  erosion 
and  in  the  carriage 
of  water  and  of  waste 
to  the  sea. 

Lateral  erosion. 
On  reaching  grade  a 
river  ceases  to  scour 
its  bed,  and  it  does 
not  again  begin  to  do 
so  until  some  change 


. 

II 


>>      73 


C        2?  «"* 

>    co 

•c?     "C^ 


®       5- 

^09 


^       ll 


82  THE  ELEMENTS  OF  GEOLOGY 

in  load  or  volume  enables  it  to  find  grade  at  a  lower  level.  On  the 
other  hand,  a  stream  erodes  its  banks  at  all  stages  in  its  history, 
and  with  graded  rivers  this  process,  called  lateral  erosion,  or 
planation,  is  specially  important.  The  current  of  a  stream  fol- 
lows the  outer  side  of  all  curves  or  bends  in  the  channel,  and  on 
this  side  it  excavates  its  bed  the  deepest  and  continually  wears 
and  saps  its  banks.  On  the  inner  side  deposition  takes  place  in 
the  more  shallow  and  slower-moving  water.  The  inner  bank  of 
bends  is  thus  built  out  while  the  outer  bank  is  worn  away.  By 
swinging  its  curves  against  the  valley  sides  a  graded  river  con- 
tinually cuts  a  wider  and  wider  floor.  The  V-valley  of  youth  is 
thus  changed  by  planation  to  a  flat-floored  valley  with  flaring 
sides  which  gradually  become  subdued  by  the  weather  to  gentle 
slopes.  While  widening  their  valleys  streams  maintain  a  con- 
stant width  of  channel,  so  that  a  wide-floored  valley  does  not 
signify  that  it  ever  was  occupied  by  a  river  of  equal  width. 

The  gradient.  The  gradients  of  graded  rivers  differ  widely. 
A  large  river  with  a  light  load  reaches  grade  on  a  faint  slope, 
while  a  smaller  stream  heavily  burdened  with  waste  requires 
a  steep  slope  to  give  it  velocity  sufficient  to  move  the  load. 

The  Platte,  a  graded  river  of  Nebraska  with  its  headwaters  in  the 
Rocky  Mountains,  is  enfeebled  by  the  semi-arid  climate  of  the  Great 
Plains  and  surcharged  with  the  waste  brought  down  both  by  its  branches 
in  the  mountains  and  by  those  whose  tracks  lie  over  the  soft  rocks 
of  the  plains.  It  is  compelled  to  maintain  a  gradient  of  eight  feet  to 
the  mile  in  western  Nebraska.  The  Ohio  reaches  grade  with  a  slope 
of  less  than  four  inches  to  the  mile  from  Cincinnati  to  its  mouth,  and 
the  powerful  Mississippi  washes  along  its  load  with  a  fall  of  but  three 
inches  per  mile  from  Cairo  to  the  Gulf. 

Other  things  being  equal,  which  of  graded  streams  will  have  the 
steeper  gradient,  a  trunk  stream  or  its  tributaries  ?  a  stream  supplied 
with  gravel  or  one  with  silt  ? 

Other  factors  remaining  the  same,  what  changes  would  occur  if  the 
Platte  should  increase  in  volume  ?  What  changes  would  occur  if  the  load 
should  be  increased  in  amount  or  in  coarseness? 


RIVERS  AND  VALLEYS 


83 


The  old  age  of  rivers.  As  rivers  pass  their  prime,  as  denuda- 
tion lowers  the  relief  of  the  region,  less  waste  and  finer  is 
washed  over  the  gentler  slopes  of  the  lowering  hills.  With 
smaller  loads  to  carry,  the  rivers  now  deepen  their  valleys  and 
find  grade  with  fainter  declivities  nearer  the  level  of  the  sea. 
This  limit  of  the  level  of  the  sea  beneath  which  they  cannot 
erode  is  known  as  baselevel.1  As  streams  grow  old  they  approach 
more  and  more  closely  to  baselevel,  although  they  are  never 
able  to  attain  it.  Some  slight  slope  is  needed  that  \vater  may 
flow  and  waste  be  transported  over  the  land.  Meanwhile  the 


FIG.  61.  Successive  Cross  Sections  of  a  Region  as  it  advances 
from  Infancy  a,  to  Old  Age  e 

relief  of  the  land  has  ever  lessened.  The  master  streams  and 
their  main  tributaries  now  wander  with  sluggish  currents  over 
the  broad  valley  floors  which  they  have  planed  away;  while 
under  the  erosion  of  their  innumerable  branches  and  the  wear 
of  the  weather  the  divides  everywhere  are  lowered  and  subdued 
to  more  and  more  gentle  slopes.  Mountains  and  high  plateaus 
are  thus  reduced  to  rolling  hills,  and  at  last  to  plains,  sur- 
mounted only  by  such  hills  as  may  still  be  unreduced  to  the 
common  level,  because  of  the  harder  rocks  of  which  they  are 
composed  or  because  of  their  distance  from  the  main  erosion 
channels.  Such  regions  of  faint  relief,  worn  down  to  near  base 
level  by  subaerial  agencies,  are  known  as  peneplains  (almost 
plains).  Any  residual  masses  which  rise  above  them  are  called 
monadnocks,  from  the  name  of  a  conical  peak  of  New  Hampshire 
which  overlooks  the  now  uplifted  peneplain  of  southern  New 
England. 

1  The  term  "baselevel"  is  also  used  to  designate  the  close  approximation  to 
sea  level  to  which  streams  are  able  to  subdue  the  land. 


84  THE  ELEMENTS  OF  GEOLOGY 

In  its  old  age  a  region  becomes  mantled  with  thick  sheets 
of  fine  and  weathered  waste,  slowly  moving  over  the  faint  slopes 
toward  the  water  ways  and  unbroken  by  ledges  of  bare  rock. 
In  other  words,  the  waste  mantle  also  is  now  graded,  and  as 
waterfalls  have  been  effaced  in  the  river  beds,  so  now  any 
ledges  in  the  wide  streams  of  waste  are  worn  away  and  covered 
beneath  smooth  slopes  of  fine  soil.  Ground  water  stands  high 
and  may  exude  in  areas  of  swamp.  In  youth  the  land  mass 
was  roughhewn  and  cut  deep  by  stream  erosion.  In  old  age 


Fi<;.  (52.   Peneplain  surmounted  by  Monadnocks,  Piedmont 
Belt,  Virginia 

From  Davis'  Elementary  Physical  Geography 

the  faint  reliefs  of  the  land  dissolve  away,  chiefly  under  the 
action  of  the  weather,  beneath  their  cloak  of  waste. 

The  cycle  of  erosion.  The  successive  stages  through  which 
a  land  mass  passes  while  it  is  being  leveled  to  the  sea  consti- 
tute together  a  cycle  of  erosion.  Each  stage  of  the  cycle  from 
infancy  to  old  age  leaves,  as  we  have  seen,  its  characteristic 
records  in  the  forms  sculptured  on  the  land,  such  as  the  shapes 
of  valleys  and  the  contours  of  hills  and  plains.  The  geologist 
is  thus  able  to  determine  by  the  land  forms  of  any  region  the 
stage  in  the  erosion  cycle  to  which  it  now  belongs,  and  know- 
ing what  are  the  earlier  stages  of  the  cycle,  to  read  something 
of  the  geological  history  of  the  region. 


RIVERS  AND  VALLEYS 


85 


Interrupted  cycles.  So  long  a  time  is  needed  to  reduce  a  land 
mass  to  baselevel  that  the  process  is  seldom  if  ever  completed 
during  a  single  uninterrupted  cycle  of  erosion.  Of  all  the  vari- 
ous interruptions  which  may  occur  the  most  important  are 
gradual  movements  of  the  earth's  crust,  by  which  a  region  is 
either  depressed  or  elevated  relative  to  sea  level. 

The  depression  of  a  region  hastens  its  old  age  by  decreasing 
the  gradient  of  streams,  by  destroying  their  power  to  excavate 
their  beds  and  cany  their  loads  to  a  degree  corresponding  to 


FIG.  63.  Young  Inner  Gorge  in  Wide  Older  Valley,  Alaska 

the  amount  of  the  depression,  and  by  lessening  the  amount  of 
work  they  have  to  do.  The  slackened  river  currents  deposit 
their  waste  in  flood  plains  which  increase  in  height  as  the  sub- 
sidence continues.  The  lower  courses  of  the  rivers  are  invaded 
by  the  sea  and  become  estuaries,  while  the  lower  tributaries  are 
cut  off  from  the  trunk  stream. 

Elevation,  on  the  other  hand,  increases  the  activity  of  all 
agencies  of  weathering,  erosion,  and  transportation,  restores  the 
region  to  its  youth,  and  inaugurates  a  new  cycle  of  erosion. 
Streams  are  given  a  steeper  gradient,  greater  velocity,  and 
increased  energy  to  carry  their  loads  and  wear  their  beds. 


86 


THE  ELEMENTS  OF  GEOLOGY 


They  cut  through  the  alluvium  of  their  flood  plains,  leaving  it 
on  either  bank  as  successive  terraces,  and  intrench  themselves 
hi  the  underlying  rock.  In  their  older  and  wider  valleys  they 
cut  narrow,  steep-walled  inner  gorges,  in  which  they  flow  swiftly 
over  rocky  floors,  broken  here  and  there  by  falls  and  rapids 

where  a  harder  layer  of  rock 
has  been  discovered.  Wind- 
ing streams  on  plains  may 
thus  incise  their  meanders  in 
solid  rock  as  the  plains  are 
gradually  uplifted.  Streams 
which  are  thus  restored  to 
their  youth  are  said  to  be 
revived. 

As  streams  cut  deeper  and 
the  valley  slopes  are  steep- 
ened, the  mantle  of  waste  of 
the  region  undergoing  eleva- 
tion is  set  in  more  rapid 
movement.  It  is  now  re- 
moved particle  by  particle 
faster  than  it  forms.  As  the 
waste  mantle  thins,  weather- 
ing attacks  the  rocks  of  the 
region  more  energetically 
until  an  equilibrium  is 
reached  again;  the  rocks 
waste  rapidly  and  their  waste  is  as  rapidly  removed. 

Dissected  peneplains.  When  a  rise  of  the  land  brings  one  cycle 
to  an  end  and  begins  another,  the  characteristic  land  forms  of 
each  cycle  are  found  together  and  the  topography  of  the  region 
is  composite  until  the  second  cycle  is  so  far  advanced  that  the 
land  forms  of  the  first  cycle  are  entirely  destroyed.  The  contrast 
between  the  land  surfaces  of  the  later  and  the  earlier  cycles  is 


Contour  Interval    50  feet 


FIG.  64.   Incised  Meanders  of  Oneota 
River,  Iowa 


RIVERS  AND  VALLEYS 


8T 


most  striking  when  the  earlier  had  advanced  to  age  and  the 

later  is  still  in  youth.    Thus  many  peneplains  which  have  been 

elevated  and  dissected  have  been  recognized  by  the  remnants 

of  their  ancient  erosion 

surfaces,  and  the  length 

of    time  which   has 

elapsed  since  their  uplift 

has  been  measured  by 

the  stage  to  which  the 

new  cycle  has  advanced. 

The  Piedmont  Belt.  As 
an  example  of  an  ancient 
peneplain  uplifted  and 
dissected  we  may  cite  the 
Piedmont  Belt,  a  broad 
upland  lying  between  the 
Appalachian  Mountains 
and  the  Atlantic  coastal 
plain.  The  surface  of  the 
Piedmont  is  gently  rolling. 
The  divides,  which  are 
often  smooth  areas  of  con- 
siderable width,  rise  to  a 
common  plane,  and  from 
them  one  sees  in  every 
direction  an  even  sky  line 
except  where  in  places 
some  lone  hill  or  ridge  may 
lift  itself  above  the  general 
level  (Fig.  62).  The  sur- 
face is  an  ancient  one,  for 
the  mantle  of  residual  waste  lies  deep  upon  it,  soils  are  reddened  by 
long  oxidation,  and  the  rocks  are  rotted  to  a  depth  of  scores  of  feet. 

At  present,  however,  the  waste  mantle  is  not  forming  so  rapidly  as 
it  is  being  removed.  The  streams  of  the  upland  are  actively  engaged  in 
its  destruction.  They  flow  swiftly  in  narrow,  rock-walled  valleys  over 
rocky  beds.  -This  contrast  between  the  young  streams  and  the  aged 


FIG.  65 

Describe  the  valley  of  stream  a.  Is  it  young  or 
old  ?  How  does  the  valley  of  b  differ  from  that 
of  a?  Compare  as  to  form  and  age  the  inner 
valley  of  &  with  the  outer  valley  and  with  the 
valley  of  a.  Account  for  the  inner  valley. 
Why  does  it  not  extend  to  the  upper  portion 
of  the  course  of  6  ?  Will  it  ever  do  so  ?  Draw 
longitudinal  profile  of  b,  showing  the  different 
gradient  of  upper  and  lower  portions  shown 
in  diagram.  We  may  suppose  that  a  also  has 
an  inner  valley  in  the  lower  portions  of  its 
course  not  here  seen.  As  the  inner  valley  of 
tributary  c  extends  headward  it  may  invade 
the  valley  of  a  before  the  inner  valley  of  a 
has  worked  upstream  to  the  area  seen  in  the 
diagram.  With  what  results  ? 


88 


THE  ELEMENTS  OF  GEOLOGY 


surface  which  they  are  now  so  vigorously  dissecting  can  only  be 
explained  by  the  theory  that  the  region  once  stood  lower  than  at 
present  and  has  recently  been  upraised.  If  now  we  imagine  the  valleys 
refilled  with  the  waste  which  the  streams  have  swept  away,  and  the 


FIG.  66.   Dissected  Peneplain  of  Southern  New  England 

upland  lowered,  we  restore  the  Piedmont  region  to  the  condition  in 
which  it  stood  before  its  uplift  and  dissection,  —  a  gently  rolling  plain, 
surmounted  here  and  there  by  isolated  hills  and  ridges. 

The  surface  of  the  ancient  Piedmont  plain,  as  it  may  be  restored 
from  the  remnants  of  it  found  on  the  divides,  is  not  in  accordance  with 
the  structures  of  the  country  rocks.  Where  these  are  exposed  to  view 
they  are  seen  to  be  far  from  horizontal.  On  the  walls  of  river  gorges 
they  dip  steeply  and  in  various  directions  and  the  streams  flow  over 

M 


FIG.  07.    Section  in  Piedmont  Belt 
M,  a  monadnock 

their  upturned  edges.  As  shown  in  Figure  67,  the  rocks  of  the  Piedmont 
have  been  folded  and  broken  and  tilted. 

It  is  not  reasonable  to  believe  that  when  the  rocks  of  the  Piedmont 
were  thus  folded  and  otherwise  deformed  the  surface  of  the  region  was 
a  plain.  The  upturned  layers  have  not  always  stopped  abruptly  at  the 
even  surface  of  the  Piedmont  plain  which  now  cuts  across  them.  They 
are  the  bases  of  great  folds  and  tilted  blocks  which  must  once  have 


RIVERS  AND  VALLEYS 


89 


risen  high  in' air.  The  complex  and  disorderly  structures  of  the  Pied- 
mont rocks  are  those  seen  in  great  mountain  ranges,  and  there  is  every 
reason  to  believe  that  these  rocks  after  their  deformation  rose  to  moun- 
tain height. 

The  ancient  Piedmont  plain  cuts  across  these  upturned  rocks  as 
independently  of  their  structure  as  the  even  surface  of  the  sawed  stump 
of  some  great  tree  is  independent  of  the  direction  of  its  fibers.  Hence' 


FIG.  68.   The  Area  of  the  Laurentian  Peneplain  (shaded) 

the  Piedmont  plain  as  it  was  before  its  uplift  was  not  a  coastal  plain 
formed  of  strata  spread  in  horizontal  sheets  beneath  the  sea  and  then 
uplifted ;  nor  was  it  a  structural  plain,  due  to  the  resistance  to  erosion 
of  some  hard,  flat-lying  layer  of  rock.  Even  surfaces  developed  on 
rocks  of  discordant  structure,  such  as  the  Piedmont  shows,  are  produced 
by  long  denudation,  and  we  may  consider  the  Piedmont  as  a  peneplain 
formed  by  the  wearing  down  of  mountain  ranges,  and  recently  uplifted. 

The  Laurentian  peneplain.    This  is  the  name  given  to  a 
denuded  surface  on  very  ancient  rocks  which  extends  from  the 


90  THE  ELEMENTS  OF  GEOLOGY 

Arctic  Ocean  to  the  St.  Lawrence  Eiver  and  Lake  Superior,  with 
small  areas  also  in  northern  Wisconsin  and  New  York.  Through- 
out this  U-shaped  area,  which  incloses  Hudson  Bay  within  its 
arms,  the  country  rocks  have  the  complicated  and  contorted  struc- 
tures which  characterize  mountain  ranges  (see  Fig.  179,  p.  211). 
But  the  surface  of  the  area  is  by  110  means  mountainous.  The 
sky  line  when  viewed  from  the  divides  is  unbroken  by  moun- 
tain peaks  or  rugged  hills.  The  surface  of  the  arm  west  of 
Hudson  Bay  is  gently  undulating  and  that  of  the  eastern  arm 
lias  been  roughened  to  low-rolling  hills  and  dissected  in  places 
by  such  deep  river  gorges  as  those  of  the  Ottawa  and  Saguenay. 
This  immense  area  may  be  regarded  as  an  ancient  peneplain 
truncating  the  bases  of  long-vanished  mountains  and  dissected 
after  elevation. 

In  the  examples  cited  the  uplift  has  been  a  broad  one  and  to 
comparatively  little  height.  Where  peneplains  have  been  uplifted 
to  great  height  and  have  since  been  well  dissected,  and  where 
they  have  been  upfolded  and  broken  and  uptilted,  their  recog- 
nition becomes  more  difficult.  Yet  recent  observers  have  found 
evidences  of  ancient  lowland  surfaces  of  erosion  on  the  summits 
of  the  Allegheny  ridges,  the  Cascade  Mountains  (Fig.  69),  and 
the  western  slope  of  the  Sierra  Nevadas. 

The  southern  Appalachian  region.  We  have  here  an  example  of  an 
area  the  latter  part  of  whose  geological  history  may  be  deciphered  by 
means  of  its  land  forms.  The  generalized  section  of  Figure  70,  which 
passes  from  west  to  east  across  a  portion  of  the  region  in  eastern  Ten- 
nessee, shows  on  the  west  a  part  of  the  broad  Cumberland  plateau.  On 
the  east  is  a  roughened  upland  platform,  from  which  rise  in  the  distance 
the  peaks  of  the  Great  Smoky  Mountains.  The  plateau,  consisting  of 
strata  but  little  changed  from  their  original  flat-lying  attitude,  and  the 
platform,  developed  on  rocks  of  disordered  structure  made  crystalline 
by  heat  and  pressure,  both  stand  at  the  common  level  of  the  line 
ab.  They  are  separated  by  the  Appalachian  valley,  forty  miles  wide, 
cut  in  strata  which  have  been  folded  and  broken  into  long  narrow 
blocks.  The  valley  is  traversed  lengthwise  by  long,  low  ridges,  the 


92  THE  ELEMENTS  OF  GEOLOGY 

outcropping  edges  of  the  harder  strata,  which  rise  to  about  the  same 
level,  —  that  of  the  line  cd.  Between  these  ridges  stretch  valley  low- 
lands at  the  level  ef,  excavated  in  the  weaker  rocks,  while  somewhat 
below  them  lie  the  channels  of  the  present  streams  now  busily  engaged 
in  deepening  their  beds. 

The  valley  lowlands.  Were  they  planed  by  graded  or  ungraded 
streams?  Have  the  present  streams  reached  grade?  Why  did  the 
streams  cease  widening  the  floors  of  the  valley  lowlands?  How  long 


FIG.  70.   Generalized  Section  of  the  Southern  Appalachian  Region  in 
Eastern  Tennessee 

since?  When  will  they  begin  anew  the  work  of  lateral  planation? 
What  effect  will  this  have  on  the  ridges  if  the  present  cycle  of  erosion 
continues  long  uninterrupted? 

The  ridges  of  the  Appalachian  valley.  Why  do  they  stand  above  the 
valley  lowlands  ?  Why  do  their  summits  lie  in  about  the  same  plane  ? 
Refilling  the  valleys  intervening  between  these  ridges  with  the  material 
removed  by  the  streams,  what  is  the  nature  of  the  surface  thus  restored  ? 
Does  this  surface  cd  accord  with  the  rock  structures  on  which  it 
has  been  developed  ?  How  may  it  have  been  made  ?  At  what  height 
did  the  land  stand  then,  compared  with  its  present  height  ?  What  eleva- 
tions stood  above  the  surface  cdl  Why?  What  name  may  you  use  to 
designate  them  ?  How  does  the  length  of  time  needed  to  develop  the 
surface  cd  compare  with  that  needed  to  develop  the  valley  lowlands? 

The  platform  and  plateau.  Why  do  they  stand  at  a  common  level 
ab  ?  Of  what  surface  may  they  be  remnants  ?  Is  it  accordant  with 
the  rock  structure  ?  How  was  it  produced  ?  What  unconsumed  masses 
overlooked  it?  Did  the  rocks  of  the  Appalachian  valley  stand  above 
this  surface  when  it  was  produced?  Did  they  then  stand  below  it? 
Compare  the  time  needed  to  develop  this  surface  with  that  needed  to 
develop  cd.  Which  surface  is  the  older? 

.  How  many  cycles  of  erosion  are  represented  here  ?  Give  the  erosion 
history  of  the  region  by  cycles,  beginning  with  the  oldest,  the  work  done 
in  each  and  the  work  left  undone,  what  brought  each  cycle  to  a  close, 
and  how  long  relatively  it  continued. 


CHAPTER  IV 
RIVER  DEPOSITS 

The  characteristic  features  of  river  deposits  and  the  forms 
which  they  assume  may  be  treated  under  three  heads :  (1)  valley 
deposits,  (2)  basin  deposits,  and  (3)  deltas. 

VALLEY  DEPOSITS 

Flood  plains  are  the  surfaces  of  the  alluvial  deposits  which 
streams  build  along  their  courses  at  times  of  flood.  A  swift 
current  then  sweeps  along  the  channel,  while  a  shallow  sheet 
of  water  moves  slowly  over  the  flood  plain,  spreading  upon  it  a 
thin  layer  of  sediment.  It  has  been  estimated  that  each  inun- 
dation of  the  Nile  leaves  a  layer  of  fertilizing  silt  three  hun- 
dredths  of  an  inch  thick  over  the  flood  plain  of  Egypt. 

Flood  plains  may  consist  of  a  thin  spread  of  alluvium  over 
the  flat  rock  floor  of  a  valley  which  is  being  widened  by  the 
lateral  erosion  of  a  graded  stream  (Fig.  60).  Flood-plain  de- 
posits of  great  thickness  may  be  built  by 
aggrading  rivers  even  in  valleys  whose 
rock  floors  have  never  been  thus  widened 
fFiff  368")  •FlGl  ^*  Cross  Section  of 

a  Flood  Plain 

A  cross  section  of  a  flood  plain  (Fig.  71) 

shows  that  it  is  highest  next  the  river,  sloping  gradually  thence 
to  the  valley  sides.  These  wide  natural  embankments  are  due 
to  the  fact  that  the  river  deposit  is  heavier  near  the  bank,  where 
the  velocity  of  the  silt-laden  channel  current  is  first  checked  by 
contact  with  the  slower-moving  overflow. 

93 


94 


THE   ELEMENTS  OF  GEOLOGY 


Thus  banked  off  from  the  stream,  the  outer  portions  of  a 
flood  plain  are  often  ill-drained  and  swampy,  and  here  vegetal 

deposits,  such  as  peat,  may 
be   interbedded   with    river 

silts. 

A  map  of  a  wide  flood  plain, 
such  as  that  of  the  Mississippi 
or  the  Missouri  (Fig.  77),  shows 
that  the  courses  of  the  tribu- 
taries on  entering  it  are  de- 
flected downstream.  Why? 

The  aggrading  streams 
by  which  flood  plains  are 
constructed  gradually  build 
their  immediate  banks  and 
beds  to  higher  and  higher 
levels,  and  therefore  find  it 
easy  at  times  of  great  floods 
to  break  their  natural  em- 
bankments and  take  new 
courses  over  the  plain.  In 
this  way  they  aggrade  each 
portion  of  it  in  turn  by 
means  of  their  shifting 
channels. 

Braided  channels.  A 
river  actively  engaged  in 
aggrading  its  valley  with 
coarse  waste  builds  a  flood 


plain  of  comparatively  steep 


FIG.  72.  Waste-Filled  Valley  and  Braided 
Channels  of  the  Upper  Mississippi 

gradient  and  often  flows  down  it  in  a  fairly  direct  course  and 
through  a  network  of  braided  channels.  From  time  to  time 
a  channel  becomes  choked  with  waste,  and  the  water  no  longer 
finding  room  in  it  breaks  out  and  cuts  and  builds  itself  a  new 


RIVER  DEPOSITS 


95 


FIG.  73.   Terraced  Valley  of  River  in  Central  Asia 

way  which  reunites  down  valley  with  the  other  channels.  Thus 
there  becomes  established  a  network  of  ever-changing  channels 
inclosing  low  islands  of  sand  and  gravel 


FIG.  74.   Terraces  carved  in  Alluvial  Deposits 
Modified  after  Davis 

Which  is  the  older,  the  rock  floor  of  the  valley  or  the  river  deposits  which 
fill  it?  "What  are  the  relative  ages  of  terraces  a,  6,  a,  and  e?  It  will 
be  noted  that  the  remnants  of  the  higher  flood  plains  have  not  been 
swept  away  by  the  meandering  river,  as  it  swung  from  side  to  side  of 
the  valley  at  lower  levels,  because  they  have  been  defended  by  ledges 
of  hard  rock  in  the  projecting  spurs  of  the  initial  valley.  The  stream 
has  encountered  such  defending  ledges  at  the  points  marked  d 


96 


THE  ELEMENTS  OF  GEOLOGY 


Terrace^.  While  aggrading  streams  thus  tend  to  shift  their 
channels,  degrading  streams,  on  the  contrary,  become  more  and 
more  deeply  intrenched  in  their  valleys.  It  often  occurs  that  a 

stream,  after  having 
built  a  flood  plain, 
ceases  to  aggrade  its  bed 
because  of  a  lessened 


FIG.  75.   River  Terraces  of  Rock  covered 
with  Alluvium 


load  or  for  other  reasons, 
such  as  an  uplift  of  the 

c,  recent  flood  plain  of  the  river.    To  what  pro-  •  i     v 

cesses  is  it  due?    Account  for  the  alluvium  at  ieglon>    c 

a  and  b  and  on  the  opposite  side  of  the  valley  at  stead    to   degrade  it.     It 

the  same  levels.   Which  is  the  older?   Account  T  ,*,  •    •      -,  a       -, 

for  the  flat  rock  floors'  on  which  these  deposits  leaves  the  Original  flood 

of  alluvium  rest.    Give  the  entire  history  which  plain    Ollt     of    reach    of 
may  be  read  in  the  section.  .,       ,  .    , 

even  the  highest  floods. 

When  again  it  reaches  grade  at  a  lower  level  it  produces  a  new 
flood  plain  by  lateral  erosion  in  the  older  deposits,  remnants  of 
which  stand  as  terraces  on  one  or  both  sides  of  the  valley.  In 
this  way  a  valley  may  be  lined  with  a  succession  of  terraces  at 
different  levels,  each  level  representing  an  abandoned  flood  plain. 
Meanders.  Valleys  aggraded  with  fine  waste  form  well-nigh 
level  plains  over  which  streams  wind  from  side  to  side  of  a 
direct  course  in  sym- 
metric bends  known 
as  meanders,  from  the 
name  of  a  winding 
river  of  Asia  Minor. 
The  giant  Mississippi 


FIG.  76.    Development  of  a  Meander 


has    developed     mean-    The  dotted  line  in  a,  b,  and  c  shows  the  stage  pre- 


ders  with  a  radius  of 


ceding  that  indicated  by  the  unbroken  line 


one  and  one  half  miles,  but  a  little  creek  may  display  on  its 
meadow  as  perfect  curves  only  a  rod  or  so  in  radius.  On  the 
flood  plain  of  either  river  or  creek  we  may  find  examples  of  the 
successive  stages  in  the  development  of  the  meander,  from  its 
beginning  in  the  slight  initial  bend  sufficient  to  deflect  the 


RIVER  DEPOSITS 


97 


current  against  the 
outer  side.  Eroding 
here  and  depositing 
on  the  inner  side  of 
the  bend,  it  gradually 
reaches  first  the 
open  bend  (Fig.  7  6,  a) 
whose  width  and 
length  are  not  far 
from  equal,  and  later 
that  of  the  horseshoe 
meander  (Fig.  76,  V) 
whose  diameter 
transverse  to  the 
course  of  the  stream 
is  much  greater  than 
that  parallel  with  it. 
Little  by  little  the 
neck  of  land  project- 
ing into  the  bend  is 
narrowed,  until  at 
last  it  is  cut  through 
and  a  "cut-off"  is 
established.  The  old 
channel  is  now  silted 


FIG.  77.  Map  of  a  Portion  of  the  Flood  Plain  of 
the  Missouri  River 


up  at  both  ends  and   Each  gmall  square  represents  one  square  mile.   How 


becomes  a  crescentic 
lagoon  (Fig.  76,  c), 
or  oxbow  lake,  which 
fills  gradually  to  an 
arc-shaped  shallow 
depression. 

Flood  plains  characteristic  of  mature  rivers.    On  reaching 
grade  a  stream  planes  a  flat  floor  for  its  continually  widening 


wide  is  the  flood  plain  of  the  Missouri  ?  How  wide 
is  the  flood  plain  of  the  Big  Sioux?  Why  is  the 
latter  river  deflected  down  valley  on  entering  the 
flood  plain  of  its  master  stream  ?  How  do  the  mean- 
ders of  the  two  rivers  compare  in  size  ?  How  does 
the  width  of  each  flood  plain  compare  with  the 
width  of  the  belt  occupied  hy  the  meanders  of  the 
river?  Do  you  find  traces  of  any  former  channels? 


98 


THE  ELEMENTS  OF  GEOLOGY 


valley.  Ever  cutting  011  the  outer  bank  of  its  curves,  it  deposits 
on  the  inner  bank  scroll-like  flood-plain  patches  (Fig.  60).  For  a 
while  the  valley  bluffs  do  not  give  its  growing  meanders  room 
to  develop  to  their  normal  size,  but  as  planation  goes  on,  the 
bluffs  are  driven  back  to  the  full  width  of  the  meander  belt 
and  still  later  to  a  width  which  gives  room  for  broad  stretches 
of  flood  plain  on  either  side  (Fig.  77). 

Usually  a  river  first  attains  grade  near  its  mouth,  and  here 
first  sinks  its  bed  to  near  baselevel.  Extending  its  graded 

course  upstream  by 
cutting  away  bar- 
rier after  barrier,  it 
comes  to  have  a 
widened  and  mature 
valley  over  its  lower 
course,  while  its 
young  headwaters 
are  still  busily  erod- 
ing their  beds.  Its 
ungraded  branches 
may  thus  bring 
down  to  its  lower 
course  more  waste 
than  it  is  competent  to  carry  on  to  the  sea,  and  here  it  aggrades 
its  bed  and  builds  a  flood  plain  in  order  to  gain  a  steeper  gra- 
dient and  velocity  enough  to  transport  its  load. 

As  maturity  is  past  and  the  relief  of  the  land  is  lessened,  a 
smaller  and  smaller  load  of  waste  is  delivered  to  the  river.  It 
now  has  energy  to  spare  and  again  degrades  its  valley,  excavat- 
ing its  former  flood  plains  and  leaving  them  in  terraces  on  either 
side,  and  at  last  in  its  old  age  sweeping  them  away. 

Alluvial  cones  and  fans.  In  hilly  and  mountainous  countries 
one  often  sees  on  a  valley  side  a  conical  or  fan-shaped  deposit  of 
waste  at  the  mouth  of  a  lateral  stream.  The  cause  is  obvious : 


Fio.  78.    Alluvial  Cones,  Wyoming 


RIVER  DEPOSITS 


99 


the  young  branch  has  not  been  able  as  yet  to  wear  its  bed  to 
accordant  level  with  the  already  deepened  valley  of  the  master 
stream.  It  therefore  builds  its  bed  to  grade  at  the  point  of  junc- 
ture by  depositing  here  its  load  of  waste,  —  a  load  too  heavy  to 
be  carried  along  the  more 
gentle  profile  of  the  trunk 
valley. 

Where  rivers  descend  from  a 
mountainous  region  upon  the 
plain  they  may  build  alluvial 
fans  of  exceedingly  gentle  slope. 
Thus  the  rivers  of  the  west- 
ern side  of  the  Sierra  Nevada 
Mountains  have  spread  fans 
with  a  radius  of  as  much  as 
forty  miles  and  a  slope  too 
slight  to  be  detected  without 
instruments,  where  they  leave 
the  rock-cut  canyons  in  the 
mountains  and  descend  upon 
the  broad  central  valley  of 
California. 

As  a  river  flows  over 
its  fan  it  commonly  divides 
into  a  branehwork  of  shift- 
ing channels  called  distribii- 
taries,  since  they  lead  off  the 
water  from  the  main  stream. 


FIG.  79.   Tributaries  and  Distributaries 
of  a  Fan-Building  Stream 


In  this  way  each  part  of  the  fan  is  aggraded  and  its  symmetric 
form  is  preserved. 

Piedmont  plains.  Mountain  streams  may  build  their  confluent 
fans  into  widespread  piedmont  (foot  of  the  mountain)  alluvial 
plains.  These  are  especially  characteristic  of  arid  lands,  where 
the  streams  wither  as  they  flow  out  upon  the  thirsty  lowlands 
and  are  therefore  compelled  to  lay  down  a  large  portion  of  their 


100         THE  ELEMENTS  OF  GEOLOGY 

load.  In  humid  climates  mountain-born  streams  are  usually 
competent  to  carry  their  loads  of  waste  on  to  the  sea,  and  have 
energy  to  spare  to  cut  the  lower  mountain  slopes  into  foothills. 
In  arid  regions  foothills  are  commonly  absent  and  the  ranges 
rise,  as  from  pedestals,  above  broad,  sloping  plains  of  stream-laid 
waste. 

The  High  Plains.  The  rivers  which  flow  eastward  from  the  Rocky 
Mountains  have  united  their  fans  in  a  continuous  sheet  of  waste  which 
stretches  forward  from  the  base  of  the  mountains  for  hundreds  of  miles 
and  in  places  is  five  hundred  feet  thick  (Fig.  80).  That  the  deposit  was 
made  in  ancient  times  on  land  and  not  in  the  sea  is  proved  by  the 


FIG.  80.   Section  from  the  Rocky  Mountains  Eastward 
River  deposits  dotted 

remains  which  it  contains  of  land  animals  and  plants  of  species  now 
extinct.  That  it  was  laid  by  rivers  and  not  by  fresh-water  lakes  is  shown 
by  its  structure.  Wide  stretches  of  flat-lying  clays  and  sands  are  inter- 
rupted by  long,  narrow  belts  of  gravel  which  mark  the  channels  of  the 
ancient  streams.  Gravels  and  sands  are  often  cross  bedded,  and  their 
well-worn  pebbles  may  be  identified  with  the  rocks  of  the  mountains. 
After  building  this  sheet  of  waste  the  streams  ceased  to  aggrade  and 
began  the  work  of  destruction.  Large  uneroded  remnants,  their  sur- 
faces flat  as  a  floor,  remain  as  the  High  Plains  of  western  Kansas  and 
Nebraska. 

River  deposits  in  subsiding  troughs.  To  a  geologist  the  most 
important  river  deposits  are  those  which  gather  in  areas  of 
gradual  subsidence ;  they  are  often  of  vast  extent  and  immense 
thickness,  and  such  deposits  of  past  geological  ages  have  not 
infrequently  been  preserved,  with  all  their  records  of  the  times 
in  which  they  were  built,  by  being  carried  below  the  level  of 
the  sea,  to  be  brought  to  light  by  a  later  uplift.  On  the  other 
hand,  river  deposits  which  remain  above  baselevels  of  erosion 
are  swept  away  comparatively  soon. 


RIVER  DEPOSITS 


The  Great  Valley  of  California  is  a  monotonously  level  plain  of  great 
fertility,  four  hundred  miles  in  length  and  fifty  miles  in  average  width, 
built  of  waste  swept  down  by  streams  from  the  mountain  ranges  which 
inclose  it,  —  the  Sierra  Nevada  on  the  east  and  the  Coast  Range  on  the 
west.  On  the  waste  slopes  at  the  foot  of  the  bordering  hills  coarse 
gravels  and  even  bowlders  are  left,  while  over  the  interior  the  slow- 
flowing  streams  at  times  of  flood  spread  wide  sheets  of  silt.  Organic 
deposits  are  now  forming  by  the  decay  of  vegetation  in  swampy  tule 
(reed)  lands  and  in  shallow  lakes  which  occupy  depressions  left  by  the 
aggrading  streams. 

Deep  borings  show  that  this  great  trough  is  rilled  to  a  depth  of  at 
least  two  thousand  feet  below  sea  level  with  recent  un consolidated 
sands  and  silts  containing  logs  of  wood  and  fresh-water  shells.  These 
are  land  deposits,  and  the  absence  of  any  marine  deposits  among  them 
proves  that  the  region  has  not  been  invaded  by  the  sea  since  the 
accumulation  began.  It  has  therefore  been  slowly  subsiding  and  its 
streams,  although  continually  carried  below  grade,  have  yet  been  able 
to  aggrade  the  surface  as  rapidly  as  the  region  sank,  and  have  main- 
tained it,  as  at  present,  slightly  above  sea  level. 

The  Indo-Gangetic  Plain,  spread  by  the  Brahmaputra,  the  Ganges,  and 
the  Indus  river  systems,  stretches  for  sixteen  hundred  miles  along  the 
southern  base  of  the  Himalaya  Mountains  and  occupies  an  area  of 
three  hundred  thousand  square  miles  (Fig.  342).  It  consists  of  the 
flood  plains  of  the  master  streams  and  the  confluent  fans  of  the  tribu- 
taries which  issue  from  the  mountains  on  the  north.  Large  areas  are 
subject  to  overflow  each  season  of  flood,  and  still  larger  tracts  mark 
abandoned  flood  plains  below  which  the  rivers  have  now  cut  their  beds. 
The  plain  is  built  of  far-stretching  beds  of  clay,  penetrated  by  streaks 
of  sand,  and  also  of  gravel  near  the  mountains.  Beds  of  impure  peat 
occur  in  it,  and  it  contains  fresh-water  shells  and  the  bones  of  land 
animals  of  species  now  living  in  northern  India.  At  Lucknow  an 
artesian  well  was  sunk  to  one  thousand  feet  below  sea  level  without 
reaching  the  bottom  of  these  river-laid  sands  and  silts,  proving  a  slow 
subsidence  with  which  the  aggrading  rivers  have  kept  pace. 

Warped  valleys.  It  is  not  necessary  that  an  area  should  sink 
below  sea  level  in  order  to  be  filled  with  stream-swept  waste. 
High  valleys  among  growing  mountain  ranges  may  suffer 
warping,  or  may  be  blockaded  by  rising  mountain  folds 


THE  ELEMENTS  OF  GEOLOGY 

athwart  them.  Where  the  deformation  is  rapid  enough,  the 
river  may  be  ponded  and  the  valley  filled  with  lake-laid  sedi- 
ments. Even  when  the  river  is  able  to  maintain  its  right  of 
way  it  may  yet  have  its  declivity  so  lessened  that  it  is  com- 
pelled to  aggrade  its  course  continually,  filling  the  valley  with 
river  deposits  which  may  grow  to  an  enormous  thickness. 

Behind  the  outer  ranges  of  the  Himalaya  Mountains  lie  several  waste- 
filled  valleys,  the  largest  of  which  are  Kashmir  and  Nepal,  the  former 
being  an  alluvial  plain  about  as  large  as  the  state  of  Delaware.  The 
rivers  which  drain  these  plains  have  already  cut  down  their  outlet 
gorges  sufficiently  to  begin  the  task  of  the  removal  of  the  broad  accu- 
mulations which  they  have  brought  in  from  the  surrounding  mountains. 
Their  present  flood  plains  lie  as  much  as  some  hundreds  of  feet  below 
wide  alluvial  terraces  which  mark  their  former  levels.  Indeed,  the 
horizontal  beds  of  the  Hundes  Valley  have  been  trenched  to  the  depth 
of  nearly  three  thousand  feet  by  the  Sutlej  River.  These  deposits  are 
recent  or  subrecent,  for  there  have  been  found  at  various  levels  the 
remains  of  land  plants  and  land  and  fresh-water  shells,  and  in  some 
the  bones  of  such  animals  as  the  hyena  and  the  goat,  of  species  or  of 
genera  now  living.  Such  soft  deposits  cannot  be  expected  to  endure 
through  any  considerable  length  of  future  time  the  rapid  erosion  to 
which  their  great  height  above  the  level  of  the  sea  will  subject  them. 

Characteristics  of  river  deposits.  The  examples  just  cited 
teach  clearly  the  characteristic  features  of  extensive  river  de- 

posits.   These  deposits 
consist  of  broad,  flat- 
lying   sheets    of   clay 
FIG.  81.  Cross  Section  of  Aggraded  Valley,  show-    and  fine   sand  left   by 
ing  Structure  of  River  Deposits  ^  overflow 


flood,  and  traversed  here  and  there  by  long,  narrow  strips  of 
coarse,  cross-bedded  sands  and  gravels  thrown  down  by  the 
swifter  currents  of  the  shifting  channels.  Occasional  beds  of 
muck  mark  the  sites  of  shallow  lakelets  or  fresh-water  swamps. 
The  various  strata  also  contain  some  remains  of  the  countless 
myriads  of  animals  and  plants  which  live  upon  the  surface  of 


RIVER  DEPOSITS  103 

the  plain  as  it  is  in  process  of  building.  River  shells  such  as 
the  mussel,  land  shells  such  as  those  of  snails,  the  bones  of 
tishes  and  of  such  land  animals  as  suffer  drowning  at  times 
of  flood  or  are  mired  in  swampy  places,  logs  of  wood,  and  the 
stems  and  leaves  of  plants  are  examples  of  the  variety  of  the 
remains  of  land  and  fresh-water  organisms  which  are  entombed 
in  river  deposits  and  sealed  away  as  a  record  of  the  life  of  the 
time,  and  as  proof  that  the  deposits  were  laid  by  streams  and 
not  beneath  the  sea. 

BASIN  DEPOSITS 

Deposits  in  dry  basins.  On  desert  areas  without  outlet  to  the 
sea,  as  on  the  Great  Basin  of  the  United  States  and  the  deserts 
of  central  Asia,  stream-swept  waste  accumulates  indefinitely. 
The  rivers  of  the  surrounding  mountains,  fed  by  the  rains  and 
melting  snows  of  these  comparatively  moist  elevations,  dry  and 
soak  away  as  they  come  down  upon  the  arid  plains.  They  are 
compelled  to  lay  aside  their  entire  load  of  waste  eroded  from 
the  mountain  valleys,  in  fans  which  grow  to  enormous  size, 
reaching  in  some  instances  thousands  of  feet  in  thickness. 

The  monotonous  levels  of  Turkestan  include  vast  alluvial  tracts  now 
in  process  of  building  by  the  floods  of  the  frequently  shifting  channels 
of  the  Oxus  and  other  rivers  of  the  region.  For  about  seven  hundred 
miles  from  its  mouth  in  Aral  Lake  the  Oxus  receives  no  tributaries, 
since  even  the  larger  branches  of  its  system  are  lost  in  a  network  of  dis- 
tributaries and  choked  with  desert  sands  before  they  reach  their  master 
stream.  These  aggrading  rivers,  which  have  channels  but  no  valleys, 
spread  their  muddy  floods  —  which  in  the  case  of  the  Oxus  sometimes 
equal  the  average  volume  of  the  Mississippi  —  far  and  wide  over  the 
plain,  washing  the  bases  of  the  desert  dunes. 

Play  as.  In  arid  ulterior  basins  the  central  depressions  may 
be  occupied  by  playas,  —  plains  of  fine  mud  washed  forward 
from  the  margins.  In  the  wet  season  the  playa  is  covered  with 
a  thin  sheet  of  muddy  water,  a  playa  lake,  supplied  usually  by 


104  THE  ELEMENTS  OF  GEOLOGY 

some  stream  at  flood.  In  the  dry  season  the  lake  evaporates,  the 
river  which  fed  it  retreats,  and  there  is  left  to  view  a  hard, 
smooth,  level  floor  of  sun-baked  and  sun-cracked  yellow  clay 
utterly  devoid  of  vegetation. 

In  the  Black  Rock  desert  of  Nevada  a  playa  lake  spreads  over  an 
area  fifty  miles  long  and  twenty  miles  wide.  In  summer  it  disappears ; 
the  Quinn  River,  which  feeds  it,  shrinks  back  one  hundred  miles  toward 
its  source,  leaving  an  absolutely  barren  floor  of  clay,  level  as  the  sea. 

Lake  deposits.  Regarding  lakes  as  parts  of  river  systems, 
we  may  now  notice  the  characteristic  features  of  the  deposits  in 
lake  basins.  Soundings  in  lakes  of  considerable  size  and  depth 
show  that  their  bottoms  are  being  covered  with  fine  clays.  Sand 
and  gravel  are  found  along  their  margins,  being  brought  in  by 
streams  and  worn  by  waves  from  the  shore,  but  there  are  no 
tidal  or  other  strong  currents  to  sweep  coarse  waste  out  from 
shore  to  any  considerable  distance.  Where  fine  clays  are  now 
found  on  the  land  in  even,  horizontal  layers  containing  the 
remains  of  fresh-water  animals  and  plants,  uncut  by  channels 
filled  with  cross-bedded  gravels  and  sands  and  bordered  by 
beach  deposits  of  coarse  waste,  we  may  safely  infer  the  exist- 
ence of  ancient  lakes. 

Marl.  Marl  is  a  soft,  whitish  deposit  of  carbonate  of  lime,  mingled 
often  with  more  or  less  of  clay,  accumulated  in  small  lakes  whose  feed- 
ing springs  are  charged  with  carbonate  of  lime  and  into  which  little 
waste  is  washed  from  the  land.  Such  lakelets  are  not  infrequent  on  the 
surface  of  the  younger  drift  sheets  of  Michigan  and  northern  Indiana, 
where  their  beds  of  marl —  sometimes  as  much  as  forty  feet  thick  —  are 
utilized  in  the  manufacture  of  Portland  cement.  The  deposit  results 
from  the  decay  of  certain  aquatic  plants  which  secrete  lime  carbonate 
from  the  water,  from  the  decomposition  of  the  calcareous  shells  of  tiny 
mollusks  which  live  in  countless  numbers  on  the  lake  floor,  and  in  some 
cases  apparently  from  chemical  precipitation. 

Peat.  We  have  seen  how  lakelets  are  extinguished  by  the 
decaying  remains  of  the  vegetation  which  they  support.  A 


RIVER  DEPOSITS  105 

section  of  such  a  fossil  lake  shows  that  below  the  growing 
mosses  and  other  plants  of  the  surface  of  the  bog  lies  a  spongy 
mass  composed  of  dead  vegetable  tissue,  which  passes  downward 
gradually  into  peat,  —  a  dense,  dark  brown  carbonaceous  deposit 
in  which,  to  the  unaided  eye,  little  or  no  trace  of  vegetable 


FIG.  82.   Digging  Peat,  Scotland 

structure  remains.  When  dried,  peat  forms  a  fuel  of  some  value 
and  is  used  either  cut  into  slabs  and  dried  or  pressed  into  bricks 
by  machinery.  , 

When  vegetation  decays  in  open  air  the  carbon  of  its  tissues, 
taken  from  the  atmosphere  by  the  leaves,  is  oxidized  and  re- 
turned to  it  in  its  original  form  of  carbon  dioxide.  But  decom- 
posing hi  the  presence  of  water,  as  in  a  bog,  where  the  oxygen 
of  the  air  is  excluded,  the  carbonaceous  matter  of  plants  accumu- 
lates in  deposits  of  peat. 

Peat  bogs  are  numerous  in  regions  lately  abandoned  by  glacier  ice, 
where  river  systems  are  so  immature  that  the  initial  depressions  left  in 
the  sheet  of  drift  spread  over  the  country  have  not  yet  been  drained. 
One  tenth  of  the  surface  of  Ireland  is  said  to  be  covered  with  peat,  and 


106  THE  ELEMENTS  OF  GEOLOGY 

small  bogs  abound  in  the  drift-covered  area  of  New  England  and  the 
states  lying  a8  far  west  as  the  Missouri  River.  In  Massachusetts  alone  it 
has  been  reckoned  that  there  are  fifteen  billion  cubic  feet  of  peat,  the 
largest  bog  occupying  several  thousand  acres. 

Much  larger  swamps  occur  on  the  young  coastal  plain  of  the  Atlantic 
from  New  Jersey  to  Florida.  The  Dismal  Swamp,  for  example,  in 
Virginia  and  North  Carolina  is  forty  miles  across.  It  is  covered  with  a 
dense  growth  of  water-loving  trees  such  as  the  cypress  and  black  gum. 
The  center  of  the  swamp  is  occupied  by  Lake  Drummond,  a  shallow 
lake  seven  miles  in  diameter,  with  banks  of  pure  peat,  and  still  narrow- 
ing from  the  encroachment  of  vegetation  along  its  borders. 

Salt  lakes.  In  arid  climates  a  lake  rarely  receives  sufficient 
inflow  to  enable  it  to  rise  to  the  basin  rim  and  find  an  outlet. 
Before  this  height  is  reached  its  surface  becomes  large  enough 
to  discharge  by  evaporation  into  the  dry  air  the  amount  of 
water  that  is  supplied  by  streams.  As  such  a  lake  has  no  out- 
let, the  minerals  in  solution  brought  into  it  by  its  streams 
cannot  escape  from  the  basin.  The  lake  water  becomes  more 
and  more  heavily  charged  with  such  substances  as  common 
salt  and  the  sulphates  and  carbonates  of  lime,  of  soda,  and  of 
potash,  and  these  are  thrown  down  from  solution  one  after 
another  as  the  point  of  saturation  for  each  mineral  is  reached. 
Carbonate  of  lime,  the  least  soluble  and  often  the  most  abundant 
mineral  brought  in,  is  the  first  to  be  precipitated.  As  concen- 
tration goes  on,  gypsum,  which  is  insoluble  in  a  strong  brine, 
is  deposited,  and  afterwards  common  salt.  As  the  saltness  of 
the  lake  varies  with  the  seasons  and  with  climatic  changes, 
gypsum  and  salt  are  laid  in  alternate  beds  and  are  interleaved 
with  sedimentary  clays  spread  from  the  waste  brought  in  by 
streams  at  times  of  flood.  Few  forms  of  life  can  live  in  bodies 
of  salt  water  so  concentrated  that  chemical  deposits  take  place, 
and  hence  the  beds  of  salt,  gypsum,  and  silt  of  such  lakes  are 
quite  barren  of  the  remains  of  life.  Similar  deposits  are  pre- 
cipitated by  the  concentration  of  sea  water  in  lagoons  and  arms 
of  the  sea  cut  off  from  the  ocean. 


RIVER  DEPOSITS 


107 


Lakes  Bonneville  and  Lahontan.  These  names  are  given  to  extinct 
lakes  which  once  occupied  large  areas  in  the  Great  Basin,  the  former 
in  Utah,  the  latter  in  northwestern  Nevada.  Their  records  remain  in 
various  old  beach  lines  which  they  drew  along  their  mountainous  shores 


«* 


FIG.  83.   Map  of  Lakes  Bonneville  and  Lahontan 
From  Davis'  Physical  Geography 

at  the  different  levels  at  which  they  stood,  and  in  the  deposits  of  their 
beds.  At  its  highest  stage  Lake  Bonneville,  then  one  thousand  feet 
deep,  overflowed  to  the  north  and  was  a  fresh-water  lake.  As  it  shrank 
below  the  outlet  it  became  more  and  more  salty,  and  the  Great  Salt 
Lake,  its  withered  residue,  is  now  depositing  salt  along  its  shores.  In 
its  strong  brine  lime  carbonate 
is  insoluble,  and  that  brought  al 
in  by  streams  is  thrown  down  b\ 
at  once  in  the  form  of  traver- 
tine. 

Lake  Lahontan  never  had  an 
outlet.    The  first  chemical  de- 


FIG.  84.   Section  of  Deposits  in  Beds  of 
Lakes  Bonneville  and  Lahontan 


posits  to  be  made  along  its  shores  were  deposits  of  travertine,  in  places 
eighty  feet  thick.  Its  floor  is  spread  with  fine  clays,  which  must  have 
been  laid  in  deep,  still  water,  and  which  are  charged  with  the  salts 


108  THE  ELEMENTS  OF  GEOLOGY 

absorbed  by  them  as  the  briny  water  of  the  lake  dried  away.  These 
sedimentary  clays  are  in  two  divisions,  the  upper  and  lower,  each  being 
about  one  hundred  feet  thick  (a  and  c,  Fig.  84).  They  are  separated 
by  heavy  deposits  of  well-rounded,  cross-bedded  gravels  and  sands 
(6,  Fig.  84),  similar  to  those  spread  at  the  present  time  by  the  inter- 
mittent streams  of  arid  regions.  A  similar  record  is  shown  in  the  old 
floors  of  Lake  Bonneville.  What  conclusions  do  you  draw  from  these 
facts  as  to  the  history  of  these  ancient  lakes? 

DELTAS 

In  the  river  deposits  which  are  left  above  sea  level  particles 
of  waste  are  allowed  to  linger  only  for  a  time.  From  alluvial  fans 
and  flood  plains  they  are  constantly  being  taken  up  and  swept 
farther  on  downstream.  Although  these  land  forms  may  long 
persist,  the  particles  which  compose  them  are  ever  changing. 
We  may  therefore  think  of  the  alluvial  deposits  of  a  valley  as  a 
stream  of  waste  fed  by  the  waste  mantle  as  it  creeps  and  washes 
down  the  valley  sides,  and  slowly  moving  onwards  to  the  sea. 

In  basins  waste  finds  a  longer  rest,  but  sooner  or  later  lakes 
and  dry  basins  are  drained  or  filled,  and  their  deposits,  if 
above  sea  level,  resume  their  journey  to  their  final  goal.  It  is 
only  when  carried  below  the  level  of  the  sea  that  they  are 
indefinitely  preserved. 

On  reaching  this  terminus,  rivers  deliver  their  load  to  the 
ocean.  In  some  cases  the  ocean  is  able  to  take  it  up  by  means 
of  strong  tidal  and  other  currents,  and  to  dispose  of  it  in  ways 
which  we  shall  study  later.  But  often  the  load  is  so  large,  or 
the  tides  are  so  weak,  that  much  of  the  waste  which  the  river 
brings  in  settles  at  its  mouth,  there  building  up  a  deposit  called 
the  delta,  from  the  Greek  letter  (A)  of  that  name,  whose  shape  it 
sometimes  resembles. 

Deltas  and  alluvial  fans  have  many  common  characteristics. 
Both  owe  their  origin  to  a  sudden  check  in  the  velocity  of  the 
river,  compelling  a  deposit  of  the  load ;  both  are  triangular  in 


RIVER  DEPOSITS 


109 


outline,  the  apex  pointing  upstream ;  and  both  are  traversed 
by  distributaries  which  build  up  all  parts  in  turn. 

In  a  delta  we  may  distinguish  deposits  of  two  distinct  kinds, 
—  the  submarine  and  the  subaerial  In  part  a  delta  is  built  of 
waste  brought  down  by  the  river  and  redistributed  and  spread 
by  waves  and  tides  over  the  sea  bottom  adjacent  to  the  river's 
mouth.  The  origin  of  these  deposits  is  recorded  in  the  remains 
of  marine  animals  and 
plants  which  they  con- 
tain. 

As  the  submarine 
delta  grows  near  to  the 
level  of  the  sea  the  dis- 
tributaries of  the  river 
cover  it  with  subaerial 
deposits  altogether 
similar  to  those  of  the 
flood  plain, -of  which 
indeed  the  subaerial 
delta  is  the  prolongation. 


FIG.  85.  Delta  of  the  Mississippi  River 

Here  extended  deposits  of  peat  may 
accumulate  in  swamps,  and  the  remains  of  land  and  fresh-water 
animals  and  plants  swept  down  by  the  stream  are  imbedded  in 
the  silts  laid  at  times  of  flood. 

Borings  made  in  the  deltas  of  great  rivers  such  as  the  Missis- 
sippi, the  Ganges,  and  the  Nile,  show  that  the  subaerial  portion 
often  reaches  a  surprising  thickness.  Layers  of  peat,  old  soils, 
and  forest  grounds  with  the  stumps  of  trees  are  discovered 
hundreds  of  feet  below  sea  level  In  the  Nile  delta  some  eight 
layers  of  coarse  gravel  were  found  interbedded  with  river  silts, 
and  in  the  Ganges  delta  at  Calcutta  a  boring  nearly  five  hun- 
dred feet  in  depth  stopped  in  such  a  layer. 

The  Mississippi  has  built  a  delta  of  twelve  thousand  three  hundred 
square  miles,  and  is  pushing  the  natural  embankments  of  its  chief  dis- 
tributaries into  the  Gulf  at  a  maximum  rate  of  a  mile  in  sixteen  years. 


110  THE  ELEMENTS  OF  GEOLOGY 

Muddy  shoals  surround  its  front,  shallow  lakes,  e.g.  lakes  Pontchart- 
rain  and  Borgne,  are  formed  between  the  growing  delta  and  the  old 
shore  line,  and  elongate  lakes  and  swamps  are  inclosed  between  the 
natural  embankments  of  the  distributaries. 

The  delta  of  the  Indus  River,  India,  lies  so  low  along  shore  that  a 
broad  tract  of  country  is  overflowed  by  the  highest  tides.  The  sub- 
marine portion  of  the  delta  has  been  built  to  near  sea  level  over  so  wide 
a  belt  offshore  that  in  many  places  large  vessels  cannot  come  even 
within  sight  of  land  because  of  the  shallow  water. 

A  former  arm  of  the  sea,  the  Rann  of  Cutch,  adjoining  the  delta  on 
the  east  has  been  silted  up  and  is  now  an  immense  barren  flat  of  sandy 
mud  two  hundred  miles  in  length  and  one  hundred  miles  in  greatest 
breadth.  Each  summer  it  is  flooded  with  salt  water  when  the  sea  is 
brought  in  by  strong  southwesterly  monsoon  winds,  and  the  climate 
during  the  remainder  of  the  year  is  hot  and  dry.  By  the  evaporation 
of  sea  water  the  soil  is  thus  left  so  salty  that  no  vegetation  can  grow 

upon  it,  and  in  places  beds 

^^^Bssss^-r— ___  of  salt  several  feet  in  thick- 


...    :'-••/> 


ness   have   accumulated. 
Under  like  conditions  salt 

FIG.  86.   Radial  Section  of  a  Delta  beds    of    Sreat    thickness 

have  been  formed  in  the 
This  section  of  a  delta  illustrates  the  structure  of  . 

the  platform  which  swift  streams  well  loaded     Pas^    and    are    now   ±ound 
with  coarse  waste  build  in  the  water  bodies     buried  among  the  deposits 
into  which  they  empty.    Three  members  may     of  ancient  deltas, 
be  distinguished :  the  bottom  set  beds,  a ;  the 
fore  set   beds,    b;    and   the   top  set  beds,   c. 

Account  for  the  slope  of  each  of  these.    Why  Subsidence    Of    great 

are  the  bottom  set  beds  of  the  finer  material 

and  why  do  they  extend  beyond  the  others?  deltas.  As  a  rule  great 
How  does  the  profile  of  this  delta  differ  from  deltas  ai'6  slowly  sink- 
that  of  an  alluvial  cone,  and  why? 

ing.    In  some  instances 

upbuilding  by  river  deposits  lias  gone  on  as  rapidly  as  the 
region  has  subsided.  The  entire  thickness  of  the  Ganges  delta, 
for  example,  so  far  as  it  has  been  sounded,  consists  of  deposits 
laid  in  open  air.  In  other  cases  interbedded  limestones  and 
other  sedimentary  rocks  containing  marine  fossils  prove  that  at 
times  subsidence  has  gained  on  the  upbuilding  and  the  delta 
has  been  covered  with  the  sea. 


RIVER  DEPOSITS  111 

It  is  by  gradual  depression  that  delta  deposits  attain  enor- 
mous thickness,  and,  being  lowered  beneath  the  level  of  the 
sea,  are  safely  preserved  from  erosion  until  a  movement  of  the 
earth's  crust  in  the  opposite  direction  lifts  them  to  form  part  of 
the  land.  We  shall  read  later  in  the  hard  rocks  of  our  continent 
the  records  of  such  ancient  deltas,  and  we  shall  not  be  sur- 
prised to  find  them  as  thick  as  are  those  now  building  at  the 
mouths  of  great  rivers. 

Lake  deltas.  Deltas  are  also  formed  where  streams  lose  their 
velocity  on  entering  the  still  waters  of  lakes.  The  shore  lines 
of  extinct  lakes,  such  as  Lake  Agassiz  and  Lakes  Bonneville 
and  Lahontan,  may  be  traced  by  the  heavy  deposits  at  the 
mouths  of  their  tributary  streams. 


We  have  seen  that  the  work  of  streams  is  to  drain  the  lands 
of  the  water  poured  upon  them  by  the  rainfall,  to  wear  them 
down,  and  to  carry  their  waste  away  to  the  sea,  there  to  be 
rebuilt  by  other  agents  into  sedimentary  rocks.  The  ancient 
strata  of  which  the  continents  are  largely  made  are  composed 
chiefly  of  material  thus  worn  from  still  more  ancient  lands  — 
lands  with  their  hills  and  valleys  like  those  of  to-day  —  and 
carried  by  their  rivers  to  the  ocean.  In  all  geological  times,  as 
at  the  present,  the  work  of  streams  has  been  to  destroy  the 
lands,  and  in  so  doing  to  furnish  to  the  ocean  the  materials 
from  which  the  lands  of  future  ages  were  to  be  made.  Before 
we  consider  how  the  waste  of  the  land  brought  in  by  streams 
is  rebuilt  upon  the  ocean  floor,  we  must  proceed  to  study  the 
work  of  two  agents,  glacier  ice  and  the  wind,  which  cooperate 
with  rivers  in  the  denudation  of  the  land. 


112 


CHAPTEE   V 
THE  WORK  OF  GLACIERS 

The  drift.  The  surface  of  northeastern  North  America,  as  far 
south  as  the  Ohio  and  Missouri  rivers,  is  generally  covered  by 
the  drift,  —  a  formation  which  is  quite  unlike  any  which  we 
have  so  far  studied.  A  section  of  it,  such  as  that  illustrated  in 
Figure  8  7,  shows  that  for  the  most  part  it  is  unstratified,  consisting 
of  clay,  sand,  pebbles,  and  even  large  bowlders,  all  mingled  pell- 
mell  together.  The  agent  which  laid  the  drift  is  one  which  can 
carry  a  load  of  material  of  all  sizes,  from  the  largest  bowlder  to 
the  finest  clay,  and  deposit  it  without  sorting. 

The  stones  of  the  drift  are  of  many  kinds.  The  region  from 
which  it  was  gathered  may  well  have  been  large  in  order  to 


FIG.  88.   Characteristic  Pebbles  from  the  Drift 

No.  1  has  six  facets;  Xo.  4,  originally  a  rounded  river  pebble,  has  been 
rubbed  down  to  one  flat  face ;  Nos.  3  and  5  are  battered  subangular 
fragments  faceted  on  one  side  only 

supply  these  many  different  varieties  of  rocks.  Pebbles  and 
bowlders  have  been  left  far  from  their  original  homes,  as  may 
be  seen  in  southern  Iowa,  where  the  drift  contains  nuggets  of 

113 


114 


THE  ELEMENTS  OF   GEOLOGY 


copper  brought  from  the  region  about  Lake  Superior.  The 
agent  which  laid  the  drift  is  one  able  to  gather  its  load  over  a 
large  area  and  carry  it  a  long  way. 

The  pebbles  of  the  drift  are  unlike  those  rounded  by  running 
water  or  by  waves.    They  are  marked  with  scratches.    Some 

are  angular,  many 
have  had  their  edges 
blunted,  while  others 
have  been  ground 
flat  and  smooth  011 
one  or  more  sides, 
like  gems  which 
have  been  faceted 
by  being  held  firmly 
against  the  lapi- 
dary's wheel  (Fig. 
88).  In  many  places 
the  upper  surface  of 
the  country  rock 
beneath  the  drift 
has  been  swept 
clean  of  residual 
clays  and  other 
waste.  All  rotten 
rock  has  been  planed 
away,  and  the  ledges 
of  sound  rock  to  which  the  surface  has  been  cut  down  have 
been  rubbed  smooth  and  scratched  with  long,  straight,  parallel 
lines  (Fig.  89).  The  agent  which  laid  the  drift  can  hold  sand 
and  pebbles  firmly  in  its  grasp  and  can  grind  them  against  the 
rock  beneath,  thus  planing  it  down  and  scoring  it,  while  faceting 
the  pebbles  also. 

Neither  water  nor  wind  can  do  these  things.    Indeed,  noth- 
ing like  the  drift  is  being  formed  by  any  process  now  at  work 


FIG.  89.  Smoothed  and  Scored  Rock  Surface  ex- 
posed to  View  by  the  Removal  of  Overlying 
Drift,  Iowa 


THE  WORK  OF  GLACIERS 


115 


anywhere  in  the  eastern  United  States.  To  find  the  agent  which 
has  laid  this  extensive  formation  we  must  go  to  a  region  of 
different  climatic  con- 
ditions. 

The  inland  ice  of 
Greenland.  Green- 
land is  about  fifteen 
hundred  miles  long 
and  nearly  seven  hun- 
dred miles  in  greatest 
width.  With  the  ex- 
ception of  a  narrow 
fringe  of  mountainous 
coast  land,  it  is  com- 
pletely buried  beneath 
a  sheet  of  ice,  in  shape 
like  a  vast  white 
shield,  whose  convex 
surface  rises  to  a 
height  of  nine  thou- 
sand feet  above  the 
sea.  The  few  explor- 
ers who  have  crossed 
the  ice  cap  found  it  a 
trackless  desert  desti- 
tute of  all  life  save 
such  lowly  forms  as 
the  microscopic  plant 
which  produces  the 
so-called  "  red  snow."  On  the  smooth  plain  of  the  interior  no 
rock  waste  relieves  the  snow's  dazzling  whiteness ;  no  streams 
of  running  water  are  seen ;  the  silence  is  broken  only  by  howl- 
ing storm  winds  and  the  rustle  of  the  surface  snow  which  they 
drive  before  them.  Sounding  with  long  poles,  explorers  find 


FIG.  90.  Map  of  Greenland 
Glacier  ice  covers  all  but  the  areas  shaded 


116          THE  ELEMENTS  OF  GEOLOGY 

that  below  the  powdery  snow  of  the  latest  snowfall  lie  suc- 
cessive layers  of  earlier  snows,  which  grow  more  and  more 
compact  downward,  and  at  last  have  altered  to  impenetrable 
ice.  The  ice  cap  formed  by  the  accumulated  snows  of  uncounted 
centuries  may  well  be  more  than  a  mile  in  depth.  Ice  thus 
formed  by  the  compacting  of  snow  is  distinguished  when  in 
motion  as  glacier  ice. 

The  inland  ice  of  Greenland  moves.  It  flows  with  imper- 
ceptible slowness  under  its  own  weight,  like  a  mass  of  some 
viscous  or  plastic  substance,  such  as  pitch  or  molasses  candy,  in 
all  directions  outward  toward  the  sea.  Near  the  edge  it  has  so 

thinned  that  mountain  peaks  are 
laid  bare,  these  islands  in  the  sea 
FIG.  91.  Hypothetic  Cross  Sec-  of  ice  being  known  as  nunata/cs. 

Down  the  valleys  of  the  coastal 

belt  it  drains  in  separate  streams  of  ice,  or  glaciers.  The  largest 
of  these  reach  the  sea  at  the  head  of  inlets,  and  are  therefore 
called  tide  glaciers.  Their  fronts  stand  so  deep  in  sea  water 
that  there  is  visible  seldom  more  than  three  hundred  feet  of  the 
wall  of  ice,  which  in  many  glaciers  must  be  two  thousand  and 
more  feet  high.  From  the  sea  walls  of  tide  glaciers  great  frag- 
ments break  off  and  float  away  as  icebergs.  Thus  snows  which 
fell  in  the  interior  of  this  northern  land,  perhaps  many  thou- 
sands of  years  ago,  are  carried  in  the  form  of  icebergs  to  melt 
at  last  in  the  North  Atlantic. 

Greenland,  then,  is  being  modeled  over  the  vast  extent  of 
its  ulterior  not  by  streams  of  running  water,  as  are  regions  in 
warm  and  humid  climates,  nor  by  currents  of  air,  as  are  deserts 
to  a  large  extent,  but  by  a  sheet  of  flowing  ice.  What  the  ice 
sheet  is  doing  in  the  interior  we  may  infer  from  a  study  of  the 
separate  glaciers  into  which  it  breaks  at  its  edge. 

The  smaller  Greenland  glaciers.  Many  of  the  smaller  glaciers 
of  Greenland  do  not  reach  the  sea,  but  deploy  on  plains  of  sand 
and  gravel.  The  edges  of  these  ice  tongues  are  often  as  abrupt 


THE  WORK  OF  GLACIERS 


117 


as  if  sliced  away  with  a  knife  (Fig.  92),  and  their  structure  is 
thus  readily  seen.  They  are  stratified,  their  layers  representing 
in  part  the  successive  snowfalls  of  the  interior  of  the  country. 
The  upper  layers  are  commonly  white  and  free  from  stones; 
but  the  lower  layers,  to  the  height  of  a  hundred  feet  or  more, 
are  dark  with  debris  which  is  being  slowly  carried  on.  So 
thickly  studded  with  stones  is  the  base  of  the  ice  that  it  is 


FIG.  92.  A  Greenland  Glacier 

sometimes  difficult  to  distinguish  it  from  the  rock  waste  which 
has  been  slowly  dragged  beneath  the  glacier  or  left  about  its 
edges.  The  waste  beneath  and  about  the  glacier  is  unsorted. 
The  stones  are  of  many  kinds,  and  numbers  of  them  have  been 
ground  to  flat  faces.  Where  the  front  of  the  ice  has  retreated 
the  rock  surface  is  seen  to  be  planed  and  scored  in  places  by 
the  stones  frozen  fast  in  the  sole  of  the  glacier. 

We  have  now  found  in  glacier  ice  an  agent  able  to  produce 
the  drift  of  North  America.    The  ice  sheet  of  Greenland  is  now 


118         THE  ELEMENTS  OF  GEOLOGY 

doing  what  we  have  seen  was  done  in  the  recent  past  in  our 
own  land.  It  is  carrying  for  long  distances  rocks  of  many 
kinds  gathered,  we  may  infer,  over  a  large  extent  of  country. 
It  is  laying  down  its  load  without  assortment  in  unstratified 
deposits.  It  grinds  down  and  scores  the  rock  over  which  it 
moves,  and  in  the  process  many  of  the  pebbles  of  its  load  are 
themselves  also  ground  smooth  and  scratched.  Since  this  work 
can  be  done  by  no  other  agent,  we  must  conclude  that  the 
northeastern  part  of  our  own  continent  was  covered  in  the 
recent  past  by  glacier  ice,  as  Greenland  is  to-day. 

VALLEY  GLACIERS 

The  work  of  glacier  ice  can  be  most  conveniently  studied  in 
the  separate  ice  streams  which  creep  down  mountain  valleys  in 
many  regions  such  as  Alaska,  the  western  mountains  of  the 
United  States-  and  Canada,  the  Himalayas,  and  the  Alps.  As 
the  glaciers  of  the  Alps  have  been  stvidied  longer  and  more 
thoroughly  than  any  others,  we  shall  describe  them  in  some 
detail  as  examples  of  valley  glaciers  in  all  parts  of  the  world. 

Conditions  of  glacier  formation.  The  condition  of  the  great 
accumulation  of  snow  to  which  glaciers  are  due  —  that  more 
or  less  of  each  winter's  snow  should  be  left  over  unmelted  and 
unevaporated  to  the  next  —  is  fully  met  in  the  Alps.  There  is 
abundant  moisture  brought  by  the  winds  from  neighboring  seas. 
The  currents  of  moist  air  driven  up  the  mountain  slopes  are 
cooled  by  their  own  expansion  as  they  rise,  and  the  moisture 
which  they  contain  is  condensed  at  a  temperature  at  or  below 
32°  F.,  and  therefore  is  precipitated  in  the  form  of  snow.  The 
summers  are  cool  and  their  heat  does  not  suffice  to  completely 
melt  the  heavy  snow  of  the  preceding  winter.  On  the  Alps 
the  snow  line  —  the  lower  limit  of  permanent  snow  —  is 
drawn  at  about  eight  thousand  five  hundred  feet  above  sea 
level.  Above  the  snow  line  on  the  slopes  and  crests,  where 


119 


120 


THE  ELEMENTS  OF   GEOLOGY 


these  are  not  too  steep,  the  snow  lies  the  year  round  and  gathers 
in  valley  heads  to  a  depth  of  hundreds  of  feet. 

This  is  but  a  small  fraction  of  the  thickness  to  which  snow 
would  be  piled  on  the  Alps  were  it  not  constantly  being 
drained  away.  Below  the  snow  fields  which  mantle  the  heights 
the  mountain  valleys  are  occupied  by  glaciers  which  extend  as 
much  as  a  vertical  mile  below  the  snow  line.  The  presence 

in  the  midst  of  forests  and 
meadows  and  cultivated 
fields  of  these  tongues  of 
ice,  ever  melting  and  yet 
from  year  to  year  losing 
none  of  their  bulk,  proves 
that  their  loss  is  made  good 
in  the  only  possible  way. 
They  are  fed  by  snow  fields 
above,  whose  surplus  of 
snow  they  drain  away  in 
the  form  of  ice.  The  pres- 
ence of  glaciers  below  the 
snow  line  is  a  clear  proof 
that,  rigid  and  motionless 
as  they  appear,  glaciers 
really  are  in  constant  motion 
down  valley. 

The  neve  field.  The  head  of  an  Alpine  valley  occupied  by 
a  glacier  is  commonly  a  broad  amphitheater  deeply  filled  with 
snow  (Fig.  93).  Great  peaks  tower  above  it,  and  snowy  slopes 
rise  on  either  side  on  the  flanks  of  mountain  spurs.  From  these 
heights  fierce  winds  drift  the  snows  into  the  amphitheater,  and 
avalanches  pour  in  their  torrents  of  snow  and  waste.  The  snow 
of  the  amphitheater  is  like  that  of  drifts  in  late  winter  after  many 
successive  thaws  and  freezings.  It  is  made  of  hard  grains  and 
pellets  and  is  called  n£ve.  Beneath  the  surface  of  the  neve 


FIG.  94.    Bergschrund  of  a  Glacier 
in  Colorado 


THE  WORK  OF  GLACIERS 


121 


field  and  at  its  outlet  the  granular  neve  has  been  compacted 
to  a  mass  of  porous  crystalline  ice.  Snow  has  been  changed 
to  neve,  and  neve  to  glacial  ice,  both  by  pressure,  which  drives 
the  air  from  the  interspaces  of  the  snowflakes,  and  also  by 
successive  meltings  and  freezings,  much  as  a  snowball  is  packed 
in  the  warm  hand  and  becomes  frozen  to  a  ball  of  ice. 

The  bergschrund.    The  neve  is  in  slow  motion.     It  breaks 
itself  loose  from  the  thinner  snows  about  it,  too  shallow  to  share 


FIG.  95.   Sea  Wall  of  the  Muir  Glacier,  Alaska 

its  motion,  and  from  the  rock  rim  which  surrounds  it,  forming  a 
deep  fissure  called  the  bergschrund,  sometimes  a  score  and  more 
feet  wide  (Fig.  94). 

Size  of  glaciers.  The  ice  streams  of  the  Alps  vary  in  size 
according  to  the  amount  of  precipitation  and  the  area  of  the 
neve  fields  which  they  drain.  The  largest  of  Alpine  glaciers, 
the  Aletsch,  is  nearly  ten  miles  long  and  has  an  average  width 
of  about  a  mile.  The  thickness  of  some  of  the  glaciers  of  the 
Alps  is  as  much  as  a  thousand  feet.  Giant  glaciers  more  than 
twice  the  length  of  the  longest  in  the  Alps  occur  on  the  south 
slope  of  the  Himalaya  Mountains,  which  receive  frequent 


122 


THE  ELEMENTS  OF  GEOLOGY 


precipitations  of  snow  from  moist  winds  from  the  Indian  Ocean. 
The  best  known  of  the  many  immense  glaciers  of  Alaska,  the 
Muir,  has  an  area  of  about  eight  hundred  square  miles  (Fig.  95). 


1 

234 

567 

„ 

" 

•^o-o 

-o-'^ 

\ 

</'/ 
_-*-'   / 

^c--c 

.-o^''0 

1 

bed 


FIG.  90.  Diagram  showing  Movement 
of  Row  of  Stakes  a,  set  in  a 
direct  line  across  the  surface  of  a 
glacier ;  b,  c,  and  d,  successive 
later  positions  of  the  stakes 


FIG.  97.  Diagram  showing  Movement 
of  Vertical  Row  of  Stakes  a,  set 
on  side  of  glacier 


Glacier  motion.  The  motion  of  the  glaciers  of  the  Alps  seldom 
exceeds  one  or  two  feet  a  day.  Large  glaciers,  because  of  the 
enormous  pressure  of  their  weight  and  because  of  less  marginal 
resistance,  move  faster  than  small  ones.  The  Muir  advances  at 

the  rate  of  seven 
feet  a  day,  and  some 
of  the  larger  tide 
glaciers  of  Green- 
land are  reported 
to  move  at  the  ex- 
ceptional rate  of 
fifty  feet  and  more 
in  the  same  time. 
Glaciers  move  faster 
by  day  than  by 
night,  and  in  sum- 
mer than  in  winter. 
Other  laws  of  glacier  motion  may  be  discovered  by  a  study  of 
Figures  96  and  97.  It  is  important  to  remember  that  glaciers 
do  not  slide  bodily  over  their  beds,  but  urged  by  gravity  move 
slowly  down  valley  in  somewhat  the  same  way  as  would  a 


FIG.  98.   Crevasses  of  a  Glacier,  Canada 


THE  WORK  OF  GLACIERS 


123 


stream  of  thick  mud.  Although  small  pieces  of  ice  are  brittle, 
the  large  mass  of  granular  ice  which  composes  a  glacier  acts 
as  a  viscous  substance. 


\ 


FIG.  99.  Longitudinal  Section  of  a  Portion  of  a 
Glacier,  showing  Transverse  Crevasses 


FIG.  100.  Map  View  of 
Marginal  Crevasses 


Crevasses.  Slight  changes  of  slope  in  the  glacier  bed,  and  the  differ- 
ent rates  of  motion  in  different  parts,  produce  tensions  under  which  the 
ice  cracks  and  opens  in  great  fissures  called  crevasses.  At  an  abrupt 


FIG.  101.  The  Rhone  Glacier,  showing  Radial  Crevasses,  the  Alps 


124 


THE  ELEMENTS  OF  GEOLOGY 


descent  in  the  bed  the  ice  is  shattered  into  great  fragments,  which 
unite  again  below  the  icefall.  Crevasses  are  opened  on  lines  at  right 
angles  to  the  direction  of  the  tension.  Transverse  crevasses  are  due  to  a 
convexity  in  the  bed  which  stretches  the  ice  lengthwise  (Fig.  99).  Mar- 
ginal crevasses  are  directed  upstream  and  inwards ;  radial  crevasses  are 
found  where  the  ice  stream  deploys  from  some  narrow  valley  and 
spreads  upon  some  more  open  space.  What  is  the  direction  of  the 
tension  which  causes  each  and  to  what  is  it  due?  (Figs.  100  and  101.) 

Lateral  and  medial  moraines.  The  surface  of  a  glacier  is 
striped  lengthwise  by  long  dark  bands  of  rock  debris.  Those  in 
the  center  are  called  the  medial  mo- 
raines. The  one  on  either  margin  is  a 
lateral  moraine,  and  is  clearly  formed  of 
waste  which  has  fallen  on  the  edge  of 
the  ice  from  the  valley  slopes.  A  medial 
moraine  cannot  be  formed  in  this  way, 
since  no  rock  fragments  can  fall  so  far 

out   from  the   sides.     But   following   it 
FIG.  102.   Map    View    of 

the  Junction  of  Two    up  the  glacial  stream,  one  finds  that  a 

Branches  of  a  Glacier  medial  moraine  takes  its  beginning  at 
are  repre-  the  junction  of  the  glacier  and  some 
tributary  and  is  formed  by  the  union 
of  their  two  adjacent  lateral  moraines  (Fig.  102).  Each  branch 
thus  adds  a  medial  moraine,  and  by  counting  the  number  of 
medial  moraines  of  a  trunk  stream 

~       f  ~fiit      in  . 

one  may  learn   of  how   many         [xT^C^T71??^ 
branches  it  is  composed. 


The    moraines 
sented  by  broken  lines 


Surface  moraines  appear  in  the  FlG  103  Crogs  Section  of  a 
lower  course  of  the  glacier  as  Glacier  showing  Lateral  Mo- 
ridges,  which  may  reach  the  ex-  raines  *»  *»  and  Medial  Mo- 

, .        i    i     •   1  ,  raines  m.  m 
ceptional  height   or  one  hundred 

feet.  The  bulk  of  such  a  ridge  is  ice.  It  has  been  protected 
from  the  sun  by  the  veneer  of  moraine  stuff ;  while  the  glacier 
surface  on  either  side  has  melted  down  at  least  the  distance  of 


THE  WORK  OF  GLACIERS  125 

the  height  of  the  ridge.  In  summer  the  lowering  of  the  glacial 
surface  by  melting  goes  on  rapidly.  In  Swiss  glaciers  it  has  been 
estimated  that  the  average  lowering  of  the  surface  by  melting 
and  evaporation  amounts  to  ten  feet  a  year.  As  a  moraine  ridge 
grows  higher  and  more  steep  by  the  lowering  of  the  surface  of 
the  surrounding  ice,  the  stones  of  its  cover  tend  to  slip  down 


FIG.  104.  Glacier  with  Medial  Moraines,  the  Alps 
Is  the  ice  moving  from  or  towards  the  observer? 

its  sides.    Thus  moraines  broaden,  until  near  the  terminus  of  a 
glacier  they  may  coalesce  in  a  wide  field  of  stony  waste. 

Englacial  drift.  This  name  is  applied  to  whatever  debris  is 
carried  within  the  glacier.  It  consists  of  rock  waste  fallen  on 
the  neve  and  there  buried  by  accumulations  of  snow,  and  of 
that  engulfed  in  the  glacier  where  crevasses  have  opened  beneath 
a  surface  moraine.  As  the  surface  of  the  glacier  is  lowered  by 
melting,  more  or  less  englacial  drift  is  brought  again  to  open 
air,  and  near  the  terminus  it  may  help  to  bury  the  ice  from 
view  beneath  a  sheet  of  debris. 


126  THE  ELEMENTS  OF  GEOLOGY 

The  ground  moraine.  The  drift  dragged  along  at  the  gla- 
cier's base  and  lodged  beneath  it  is  known  as  the  ground  mo- 
raine. Part  of  the  material  of  it  has  fallen  down  deep  crevasses 
and  part  has  been  torn  and  worn  from  the  glacier's  bed  and 
banks.  While  the  stones  of  the  surface  moraines  remain  as 
angular  as  when  they  lodged  on  the  ice,  many  of  those  of  the 
ground  moraine  have  been  blunted  on  the  edges  and  faceted 
and  scratched  by  being  ground  against  one  another  and  the 
rocky  bed. 

In  glaciers  such  as  those  of  Greenland,  whose  basal  layers  are  well 
loaded  with  drift  and  whose  surface  layers  are  nearly  clean,  different 
layers  have  different  rates  of  motion,  according  to  the  amount  of  drift 
with  which  they  are  clogged.  One  layer  glides  over  another,  and  the 
stones  inset  in  each  are  ground  and  smoothed  and  scratched.  Usually 
the  sides  of  glaciated  pebbles  are  more  worn  than  the  ends,  and  the 
scratches  upon  them  run  with  the  longer  axis  of  the  stone.  Why  ? 

The  terminal  moraine.  As  a  glacier  is  in  constant  motion,  it 
brings  to  its  end  all  of  its  load  except  such  parts  of  the  ground 
moraine  as  may  find  permanent  lodgment  beneath  the  ice. 
Where  the  glacier  front  remains  for  some  time  at  one  place, 
there  is  formed  an  accumulation  of  drift  known  as  the  terminal 
moraine.  In  valley  glaciers  it  is  shaped  by  the  ice  front  to  a 
crescent  whose  convex  side  is  downstream.  Some  of  the  peb- 
bles of  the  terminal  moraine  are  angular,  and  some  are  faceted  and 
scored,  the  latter  having  come  by  the  hard  road  of  the  ground 
moraine.  The  material  of  the  dump  is  for  the  most  part 
unsorted,  though  the  water  of  the  melting  ice  may  find  oppor- 
tunity to  leave  patches  of  stratified  sands  and  gravels  in  the 
midst  of  the  unstratified  mass  of  drift,  and  the  finer  material 
is  in  places  washed  away. 

Glacier  drainage.  The  terminal  moraine  is  commonly  breached 
by  a  considerable  stream,  which  issues  from  beneath  the  ice 
by  a  tunnel  whose  portal  has  been  enlarged  to  a  beautiful 


127 


128 


THE  ELEMENTS  OF  GEOLOGY 


archway  by  melting  in  the  sun  and  the  warm  air  (Fig.  107).  The 
stream  is  gray  with  silt  and  loaded  with  sand  and  gravel  washed 
from  the  ground  moraine.  "  Glacier  milk  "  the  Swiss  call  this 
muddy  water,  the  gray  color  of  whose  silt  proves  it  rock  flour 
freshly  ground  by  the  ice  from  the  unoxidized  sound  rock  of  its 


FIG.  100.   Heavy  Moraine  about  the  Terminus  of  a  Glacier  in  the 
Rocky  Mountains  of  Canada 

Account  for  the  fact  that  the  morainic  ridge  rises  considerably  ahove  the 
'surface  of  the  ice 

bed,  the  mud  of  streams  being  yellowish  when  it  is  washed 
from  the  oxidized  mantle  of  waste.  Since  glacial  streams  are 
well  loaded  with  waste  due  to  vigorous  ice  erosion,  the  valley 
in  front  of  the  glacier  is  commonly  aggraded  to  a  broad,  flat 
floor.  These  outwash  deposits  are  known  as  valley  drift. 

The  sand  brought  out  by  streams  from  beneath  a  glacier  differs  from 
river  sand  in  that  it  consists  of  freshly  broken  angular  grains.  Why? 

The  stream  derives  its  water  chiefly  from  the  surface  melting  of 
the  glacier.  As  the  ice  is  touched  by  the  rays  of  the  morning  sun  in 


THE  WORK  OF  GLACIERS 


129 


summer,  water  gathers  in  pools,  and  rills  trickle  and  unite  in  brooklets 
which  melt  and  cut  shallow  channels  in  the  blue  ice.  The  course  of 
these  streams  is  short.  Soon  they  plunge  into  deep  wells  cut  by  their 
whirling  waters  where  some  crevasse  has  begun  to  open  across  their 
path.  These  wells  lead  into  chambers  and  tunnels  by  which  sooner 
or  later  their  waters  find  way 
to  the  rock  floor  of  the  valley 
and  there  unite  in  a  subglacial 
stream. 

The  lower  limit  of  gla- 
ciers. The  glaciers  of  a  region 
do  not  by  any  means  end 
at  a  uniform  height  above 
sea  level.  Each  terminates 
where  its  supply  is  balanced 
by  melting.  Those  therefore 
which  are  fed  by  the  largest 
and  deepest  neves  and  those 
also  wrhich  are  best  protected 
from  the  sun  by  a  northward  exposure  or  by  the  depth  of  their 
inclosing  valleys  flow  to  lo\ver  levels  than  those  whose  supply 
is  less  and  whose  exposure  to  the  sun  is  greater. 

A  series  of  cold,  moist  years,  with  an  abundant  snowfall, 
causes  glaciers  to  thicken  and  advance ;  a  series  of  warm,  dry 
years  causes  them  to  wither  and  melt  back.  The  variation 
in  glaciers  is  now  carefully  observed  in  many  parts  of  the 
world.  The  Muir  glacier  has  retreated  two  miles  in  twenty 
years.  The  glaciers  of  the  Swiss  Alps  are  now  for  the  most  part 
melting  back,  although  a  well-known  glacier  of  the  eastern 
Alps,  the  Yernagt,  advanced  five  hundred  feet  in  the  year 
1900,  and  was  then  plowing  up  its  terminal  moraine. 

How  soon  would  you  expect  a  glacier  to  advance  after  its  ne've'  fields 
have  been  swollen  with  unusually  heavy  snows,  as  compared  with  the 
time  needed  for  the  flood  of  a  large  river  to  reach  its  mouth  after 
heavy  rains  upon  its  headwaters  ? 


FIG.  107.   Subglacial    Stream    issuing 
from  Tunnel  in  the  Ice,  Norway 


130 


THE  ELEMENTS  OF  GEOLOGY 


On  the  surface  of  glaciers  in  summer  time  one  may  often  see  large 
stones  supported   by  pillars  of  ice  several  feet  in  height  (Fig.  108). 
These  "  glacier  tables  "  commonly  slope  more  or  less  strongly  to  the 
south,   and    thus  may  be    used   to    indicate 
roughly  the  points  of  the  compass.     Can  you 
explain  their    formation    and    the  direction 
of  their  slope  ?    On  the  other  hand,  a  small 
and  thin   stone,   or  a   patch    of  dust,   lying 
on  the  ice,  tends  to  sink  a  few  inches  into  it. 
FIG.  108.   A  Glacier  Table    why? 

In  what  respects  is  a  valley  glacier  like  a  mountain  stream  which 
flows  out  upon  desert  plains? 

Two  confluent  glaciers  do  not  mingle  their  currents  as  do  two  con- 
fluent rivers.  What  characteristics  of  surface  moraines  prove  this  fact? 

What  effect  would  you  expect  the  laws  of  glacier  motion  to  have 
on  the  slant  of  the  sides  of  transverse  crevasses  ? 


FIG.  109.   Map  of  Malaspina  Glacier,  Alaska 

A  trunk  glacier  has  four  medial  moraines.  Of  how  many  tributaries 
is  it  composed?  Illustrate  by  diagram. 

State  all  the  evidences  which  you  have  found  that  glaciers  move. 

If  a  glacier  melts  back  with  occasional  pauses  up  a  valley,  what 
records  are  left  of  its  retreat  ? 


THE  WORK  OF  GLACIERS 


131 


PIEDMONT  GLACIEKS 

The  Malaspina  glacier.  Piedmont  (foot  of  the  mountain) 
glaciers  are,  as  the  name  implies,  ice  fields  formed  at  the  foot  of 
mountains  by  the  confluence  of  valley  glaciers.  The  Malaspina 
glacier  of  Alaska,  the  typical  glacier  of  this  kind,  is  seventy 
miles  wide  and  stretches  for  thirty  miles  from  the  foot  of  the 
Mount  Saint  Elias  range  to  the  shore  of  the  Pacific  Ocean.  The 
valley  glaciers  which  unite  and  spread  to  form  this  lake  of  ice  lie 
above  the  snow  line  and  their  moraines  are  concealed  beneath 
neve.  The  central 
area  of  the  Malas- 
pina is  also  free 
from  debris ;  but 
on  the  outer  edge 
large  quantities  of 
englacial  drift  are 


FIG.  110.   Outwash  Plain,  the  Delta  of  the 
Yahtse  River,  Alaska 


exposed  by  surface 
melting  and  form  a 
belt  of  morainic 
waste  a  few  feet 
thick  and  several 
miles  wide,  covered 
in  part  with  a  lux- 
uriant forest,  be- 
neath which  the  ice  is  in  places  one  thousand  feet  in  depth. 
The  glacier  here  is  practically  stagnant,  and  lakes  a  few  hundred 
yards  across,  which  could  not  exist  were  the  ice  in  motion  and 
broken  with  crevasses,  gather  on  their  beds  sorted  waste  from 
the  moraine.  The  streams  which  dram  the  glacier  have  cut 
their  courses  in  englacial  and  subglacial  tunnels ;  none  flow  for 
any  distance  on  the  surface.  The  largest,  the  Yahtse  River, 
issues  from  a  high  archway  in  the  ice,  —  a  muddy  torrent  one 
hundred  feet  wide  and  twenty  feet  deep,  loaded  with  sand  and 


132  THE  ELEMENTS  OF  GEOLOGY 

stones  which  it  deposits  in  a  broad  outwash  plain  (Fig.  110). 
Where  the  ice  has  retreated  from  the  sea  there  is  left  a  hum- 
mocky  drift  sheet  with  hollows  filled  with  lakelets.  These 
deposits  help  to  explain  similar  hummocky  regions  of  drift 
and  similar  plains  of  coarse,  water-laid  material  often  found  in 
the  drift-covered  area  of  the  northeastern  United  States. 

THE  GEOLOGICAL  WORK  OF  GLACIER  ICE 

The  sluggish  glacier  must  do  its  work  in  a  different  way 
from  the  agile  river.  The  mountain  stream  is  swift  and  small, 
and  its  channel  occupies  but  a  small  portion  of  the  valley. 
The  glacier  is  slow  and  big;  its  rate  of  motion  may  be 
less  than  a  millionth  of  that  of  running  water  over  the  same 
declivity,  and  its  bulk  is  proportionately  large  and  fills  the 
valley  to  great  depth.  Moreover,  glacier  ice  is  a  solid  body 
plastic  under  slowly  applied  stresses,  while  the  water  of  rivers 
is  a  nimble  fluid. 

Transportation.  Valley  glaciers  differ  from  rivers  as  carriers 
in  that  they  float  the  major  part  of  their  load  upon  their  surface, 
transporting  the  heaviest  bowlder  as  easily  as  a  grain  of  sand ; 
while  streams  push  and  roll  much  of  their  load  along  their  beds, 
and  their  power  of  transporting  waste  depends  solely  upon  their 
velocity.  The  amount  of  the  surface  load  of  glaciers  is  limited 
only  by  the  amount  of  waste  received  from  the  mountain  slopes 
above  them.  The  moving  floor  of  ice  stretched  high  across  a 
valley  sweeps  along  as  lateral  moraines  much  of  the  waste 
which  a  mountain  stream  would  let  accumulate  in  talus  and 
alluvial  cones. 

While  a  valley  glacier  carries  much  of  its  load  on  top,  an  ice 
sheet,  such  as  that  of  Greenland,  is  free  from  surface  debris, 
except  where  moraines  trail  away  from  some  nunatak.  If  at  its 
edge  it  breaks  into  separate  glaciers  which  drain  down  mountain 
valleys,  these  tongues  of  ice  will  carry  the  selvages  of  waste 


THE  WORK  OF  GLACIERS  133 

common  to  valley  glaciers.  Both  ice  sheets  and  valley  glaciers 
drag  on  large  quantities  of  rock  waste  in  their  ground  moraines. 

Stones  transported  by  glaciers  are  sometimes  called  erratics. 
Such  are  the  bowlders  of  the  drift  of  our  northern  states. 
Erratics  may  be  set  down  in  an  insecure  position  on  the  melting 
of  the  ice. 

Deposit.  Little  need  be  added  here  to  what  has  already  been 
said  of  ground  and  terminal  moraines.  All  strictly  glacial 
deposits  are  unstratified.  The  load  laid  down  at  the  end  of  a 
glacier  in  the  terminal  moraine  is  loose  in  texture,  while  the 
drift  lodged  beneath  the  glacier  as  ground  moraine  is  often  an 
extremely  dense,  stony  clay,  having  been  compacted  under  the 
pressure  of  the  overriding  ice. 

Erosion.  A  glacier  erodes  its  bed  and  banks  in  two  ways, — 
by  abrasion  and  by  plucking. 

The  rock  bed  over  which  a  glacier  has  moved  is  seen  in  places 
to  have  been  abraded,  or  ground  away,  to  smooth  surfaces  which 
are  marked  by  long,  straight,  parallel  scorings  aligned  with  the 
line  of  movement  of  the  ice  and  varying  in  size  from  hair  lines  and 
coarse  scratches  to  exceptional  furrows  several  feet  deep.  Clearly 
this  work  has  been  accomplished  by  means  of  the  sharp  sand, 
the  pebbles,  and  the  larger  stones  with  which  the  base  of  the 
glacier  is  inset,  and  which  it  holds  in  a  firm  grasp  as  running 
water  cannot.  Hard  and  fine-grained  rocks,  such  as  granite  and 
quartzite,  are  often  not  only  ground  down  to  a  smooth  surface 
but  are  also  highly  polished  by  means  of  fine  rock  flour  worn 
from  the  glacier  bed. 

In  other  places  the  bed  of  the  glacier  is  rough  and  torn.  The 
rocks  have  been  disrupted  and  their  fragments  have  been  carried 
away,  —  a  process  known  as  plucking.  Moving  under  immense 
pressure  the  ice  shatters  the  rock,  breaks  off  projections,  presses 
into  crevices  and  wedges  the  rocks  apart,  dislodges  the  blocks 
into  which  the  rock  is  divided  by  joints  and  bedding  planes,  and 
freezing  fast  to  the  fragments  drags  them  on.  In  this  work  the 


134 


THE  ELEMENTS  OF  GEOLOGY 


freezing  and  thawing  of  subglacial  waters  in  any  cracks  and 
crevices  of  the  rock  no  doubt  play  an  important  part.  Pluck- 
ing occurs  especially  where  the  bed  rock  is  weak  because  of 
close  jointing.  The  product  of  plucking  is  bowlders,  while  the 
product  of  abrasion  is  fine  rock  flour  and  sand. 

Is  the  ground  moraine  of  Figure  87  due  chiefly  to  abrasion  or  to 
plucking  ? 

Roches  moutonnees  and  rounded  hills.  The  prominences  left 
between  the  hollows  due  to  plucking  are  commonly  ground 

down  and  rounded  on 
the  s.toss  side,  —  the 
side  from  which  the  ice 
advances,  —  and  some- 
times on  the  opposite, 
the  lee  side,  as  well.  In 
this  way  the  bed  rock 
often  comes  to  have  a 
billowy  surface  known 
as  roches  moutonnees 
(sheep  rocks).  Hills 
overridden  by  an  ice 
sheet  often  have  simi- 
larly rounded  contours  on  the  stoss  side,  while  on  the  lee  side 
they  may  be  craggy,  either  because  of  plucking  or  because  here 
they  have  been  less  worn  from  their  initial  profile  (Fig.  112). 

The  direction  of  glacier  movement.  The  direction  of  the  flow 
of  vanished  glaciers  and  ice  sheets  is  recorded  both  in  the  dif- 
ferences just  mentioned  in  the  profiles  of  overridden  hills  and 
also  in  the  minute  details  of  the  glacier  trail. 

Flint  nodules  or  other  small  prominences  in  the  bed  rock  are 
found  more  worn  on  the  stoss  than  on  the  lee  side,  where  indeed 
they  may  have  a  low  cone  of  rock  protected  by  them  from 
abrasion.  Cavities,  on  the  other  hand,  have  their  edges  worn  on 
the  lee  side  and  left  sharp  upon  the  stoss. 


FIG.  111.   Roches  Moutonne'es,  Bronx  Park, 
New  York 


THE  WORK  OF  GLACIERS 


135 


Surfaces  worn  and  torn  in  the  ways  which  we  have  mentioned 
are  said  to  be  glaciated.  But  it  must  not  be  supposed  that  a 
glacier  everywhere  glaciates  its  bed.  Although  in  places  it  acts 
as  a  rasp  or  as  a  pick,  in  others,  and  especially  where  its  pressure 
is  least,  as  near  the  terminus,  it  moves  over  its  bed  in  the  manner 
of  a  sled.  Instances  are  known  where  glaciers  have  advanced 
over  deposits  of  sand  and  gravel  without  disturbing  them  to 
any  notable  degree.  Like  a  river,  a  glacier  does  not  everywhere 
erode.  In  places  it 
leaves  its  bed  un- 
disturbed and  in 
places  aggrades  it 
by  deposits  of  the 
ground  moraine. 

Cirques.  Valley 
glaciers  commonly 
head,  as  we  have 
seen,  in  broad  am- 
phitheaters deeply  filled  with  snow  and  ice.  On  mountains  now 
destitute  of  glaciers,  but  whose  glaciation  shows  that  they  have 
supported  glaciers  in  the  past,  there  are  found  similar  crescentic 
hollows  with  high,  precipitous  walls  and  glaciated  floors.  Their 
floors  are  often  basined  and  hold  lakelets  whose  deep  and  quiet 
waters  reflect  the  sheltering  ramparts  of  rugged  rock  which 
tower  far  above  them.  Such  mountain  hollows  are  termed 
cirques.  As  a  powerful  spring  wears  back  a  recess  in  the  val- 
ley side  where  it  discharges,  so  the  fountain  head  of  a  glacier 
gradually  wears  back  a  cirque.  In  its  slow  movement  the  neve 
field  broadly  scours  its  bed  to  a  flat  or  basined  floor.  Mean- 
while the  sides  of  the  valley  head  are  steepened  and  driven  back 
to  precipitous  walls.  For  in  winter  the  crevasse  of  the  berg- 
schrund  which  surrounds  the  neve  field  is  filled  with  snow  and 
the  neve  is  frozen  fast  to  the  rocky  sides  of  the  valley.  In  early 
summer  the  neve  tears  itself  free,  dislodging  and  removing 


FIG.  112.    A  Glaciated  Hill,  Norway.    Sharp 
Cirque-Cut  Mountain  Peaks  in  the  Distance 


136 


THE  WORK  OF  GLACIERS 


137 


any  loosened  blocks,  and  the  open  fissure  of  the  bergschrund 
allows  frost  and  other  agencies  of  weathering  to  attack  the  un- 
protected rock.  As  cirques  are  thus  formed  and  enlarged  the 
peaks  beneath  which  they  lie  are  sharpened^  and  the  mountain 
crests  are  scalloped  and  cut  back  from  either  side  to  knife-edged 
ridges  (Figs.  113  and  93). 

In  the  western  mountains  of  the  United  States  many  cirques, 
now  empty  of  nev6  and  glacier  ice,  and  known  locally  as 
"  basins,"  testify  to  the  fact  that  in  recent  times  the  snow  line 
stood  beneath  the  levels  of  their  floors,  and  thus  far  below  its 
present  altitude. 

Glacier  troughs.  The  channel  worn  to  accommodate  the  big 
and  clumsy  glacier  differs  markedly  from  the  river  valley  cut 


FIG.  114.   A  Glacier  Trough,  Montana 

as  with  a  saw  by  the  narrow  and  flexible  stream  and  widened 
by  the  weather  and  the  wash  of  rains.  The  valley  glacier  may 
easily  be  from  one  thousand  to  three  thousand  feet  deep  and 
from  one  to  three  miles  wide.  Such  a  ponderous  bulk  of  slowly 


138          THE  ELEMENTS  OF  GEOLOGY 

moving  ice  does  not  readily  adapt  itself  to  sharp  turns  and  a 
narrow  bed.  By  scouring  and  plucking  all  resisting  edges  it  de- 
velops a  fitting  channel  with  a  wide,  flat  floor,  and  steep,  smooth 
sides,  above  which  are  seen  the  weathered  slopes  of  stream-worn 
mountain  valleys.  Since  the  trunk  glacier  requires  a  deeper 
channel  than  do  its  branches,  the  bed  of  a  branch  glacier  enters 
the  main  trough  at  some  distance  above  the  floor  of  the  latter, 


FIG.  115.   Lynn  Canal,  Alaska,  a  Fjord 

although  the  surface  of  the  two  ice  streams  may  be  accordant. 
Glacier  troughs  can  be  studied  best  where  large  glaciers  have 
recently  melted  completely  away,  as  is  the  case  in  many  valleys 
of  the  mountains  of  the  western  United  States  and  of  central 
and  northern  Europe  (Fig.  114).  The  typical  glacier  trough,  as 
shown  in  such  examples,  is  U-shaped,  with  a  broad,  flat  floor, 
and  high,  steep  walls.  Its  walls  are  little  broken  by  projecting 
spurs  and  lateral  ravines.  It  is  as  if  a  V-valley  cut  by  a  river 
had  afterwards  been  gouged  deeper  with  a  gigantic  chisel,  wid- 
ening the  floor  to  the  width  of  the  chisel  blade,  cutting  back  the 
spurs,  and  smoothing  and  steepening  the  sides.  A  river  valley 


THE  WORK  OF  GLACIERS 


139 


could  only  be  as  wide-floored  as  this  after  it  had  long  been 
worn  down  to  grade. 

The  floor  of  a  glacier  trough  may  not  be  graded ;  it  is  often 
interrupted  by  irregular  steps  perhaps  hundreds  and  even  a 
thousand  feet  in  height, 
over  which  the  stream  that 
now  drains  the  valley  tum- 
bles in  waterfalls.  Eeaches 
between  the  steps  are  often 
basined.  Lakelets  may 
occupy  hollows  excavated 
in  solid  rock,  and  other  lakes 
may  be  held  behind  terminal 
moraines  left  as  dams  across 
the  valley  at  pauses  in  the 
retreat  of  the  glacier. 

Fjords  are  glacier  troughs 
now  occupied  in  part  or  wholly 
by  the  sea,  either  because  they 
were  excavated  by  a  tide  glacier 
to  their  present  depth  below 
sea  level,  or  because  of  a  sub- 
mergence of  the  land.  Their 
characteristic  form  is  that  of  a 
long,  deep,  narrow  bay  with 


FIG.  116.  A,  V-River  Valley,  with  Valley 
of  Tributary  joining  it  at  Accordant 
Level ;  B,  the  Same  changed  after 
Long  Glaciation  to  a  Glacier  Trough 
with  Hanging  Valley 


steep  rock  walls   and   basined 

floor    (Fig.    115).     Fjords   are 

found  only  in  regions  which  have  suffered  glaciation,  such  as  Norway 

and  Alaska. 

Hanging  valleys.  These  are  lateral  valleys  which  open  on 
their  main  valley  some  distance  above  its  floor.  They  are  con- 
spicuous features  of  glacier  troughs  from  which  the  ice  has  van- 
ished; for  the  trunk  glacier  in  widening  and  deepening  its 
channel  cut  its  bed  below  the  bottoms  of  the  lateral  valleys 
(Fig.  116). 


140 


THE  ELEMENTS  OF  GEOLOGY 


Since  the  mouths  of  hanging  valleys  are  suspended  on  the 
walls  of  the  glacier  trough,  their  streams  are  compelled  to 
plunge  down  its  steep,  high  sides  in  waterfalls.  Some  of  the 
loftiest  and  most  beautiful  waterfalls  of  the  world  leap  from 
hanging  valleys, —  among  them  the  celebrated  Staubbach  of  the 
Lauterbrunnen  valley  of  Switzerland,  and  those  of  the  fjords 
of  Norway  and  Alaska  (Fig.  117). 

Hanging  valleys  are  found  also  in  river  gorges  where  the 
smaller  tributaries  have  not  been  able  to  keep  pace  with  a 

strong  master  stream  in 
cutting  down  their  beds. 
In  this  case,  however, 
they  are  a  mark  of  ex- 
treme youth ;  for,  as  the 
trunk  stream  approaches 
grade  and  its  velocity 
and  power  to  erode  its 
bed  decrease,  the  side 
streams  soon  cut  back 
their  falls  and  wear 
their  beds  at  their 
mouths  to  a  common 
level  with  that  of  the  main  river.  The  Grand  Canyon  of  the 
Colorado  must  be  reckoned  a  young  valley.  At  its  base  it  nar- 
rows to  scarcely  more  than  the  width  of  the  river,  and  yet  its 
tributaries,  except  the  very  smallest,  enter  it  at  a  common  level. 

Why  could  not  a  wide-floored  valley,  such  as  a  glacier  trough,  with 
hanging  valleys  opening  upon  it,  be  produced  in  the  normal  develop- 
ment of  a  river  valley  ? 

The  troughs  of  young  and  of  mature  glaciers.  The  features  of  a  glacier 
trough  depend  much  on  the  length  of  time  the  preexisting  valley  was 
occupied  with  ice.  During  the  infancy  of  a  glacier,  we  may  believe,  the 
spurs  of  the  valley  which  it  fills  are  but  little  blunted  and  its  bed  is 
but  little  broken  by  steps.  In  youth  the  glacier  develops  icefalls,  as  a 


FIG.  117.   Hanging  Valley  on  the  Wall  of 
a  Fjord,  Norway 


THE  WORK  OF  GLACIERS  141 

river  in  youth  develops  waterfalls,  and  its  bed  becomes  terraced  with  great 
stairs.  The  mature  glacier,  like  the  mature  river,  has  effaced  its  falls  and 
smoothed  its  bed  to  grade.  It  has  also  worn  back  the  projecting  spurs 
of  its  valley,  making  itself  a  wide  channel  with  smooth  sides.  The  bed 
of  a  mature  glacier  may  form  a  long  basin,  since  it  abrades  most  in  its 
upper  and  middle  course,  where  its  weight  and  motion  are  the  greatest. 
Near  the  terminus,  where  weight  and  motion  are  the  least,  it  erodes 
least,  and  may  instead  deposit  a  sheet  of  ground  moraine,  much  as  a 
river  builds  a  flood  plain  in  the  same  part  of  its  course  as  it  approaches 
maturity.  The  bed  of  a  mature  glacier  thus  tends  to  take  the  form  of  a 
long,  relatively  narrow  basin,  across  whose  lower  end  may  be  stretched 
the  dam  of  the  terminal  moraine.  On  the  disappearance  of  the  ice  the 
basin  is  filled  with  a  long,  narrow  lake,  such  as  Lake  Chelan  in  Wash- 
ington and  many  of  the  lakes  in  the  Highlands  of  Scotland. 

Piedmont  glaciers  apparently  erode  but  little.  Beneath  their  lake- 
like  expanse  of  sluggish  or  stagnant  ice  a  broad  sheet  of  ground 
moraine  is  probably  being  deposited. 

Cirques  and  glaciated  valleys  rapidly  lose  their  characteris- 
tic forms  after  the  ice  has  withdrawn.  The  weather  destroys 
all  smoothed,  polished,  and  scored  surfaces  which  are  not  pro- 
tected beneath  glacial  deposits.  The  oversteepened  sides  of  the 
trough  are  graded  by  landslips,  by  talus  slopes,  and  by  alluvial 
cones.  Morainic  heaps  of  drift  are  dissected  and  carried  away. 
Hanging  valleys  and  the  irregular  bed  of  the  trough  are  both 
worn  down  to  grade  by  the  streams  which  now  occupy  them. 
The  length  of  time  since  the  retreat  of  the  ice  from  a  mountain 
valley  may  thus  be  estimated  by  the  degree  to  which  the  destruc- 
tion of  the  characteristic  features  of  the  glacier  trough  has  been 
carried. 

In  Figure  104  what  characteristics  of  a  glacier  trough  do  you  notice? 
What  inference  do  you  draw  as  to  the  former  thickness  of  the  glacier  ? 

Name  all  the  evidences  you  would  expect  to  find  to  prove  the  fact 
that  in  the  recent  geological  past  the  valleys  of  the  Alps  contained  far 
larger  glaciers  than  at  present,  and  that  on  the  north  of  the  Alps  the 
ice  streams  united  in  a  piedmont  glacier  which  extended  across  the 
plains  of  Switzerland  to  the  sides  of  the  Jura  Mountains. 


142 


THE   ELEMENTS  OF   GEOLOGY 


The  relative  importance  of  glaciers  and  of  rivers.  Powerful 
as  glaciers  are,  and  marked  as  are  the  land  forms  which  they 
produce,  it  is  easy  to  exaggerate  their  geological  importance  as 
compared  with  rivers.  Under  present  climatic  conditions  they 
are  confined  to  lofty  mountains  or  polar  lands.  Polar  ice 
sheets  are  permanent  only  so  long  as  the  lands  remain  on 
which  they  rest.  Mountain  glaciers  can  stay  only  the  brief 
tune  during  which  the  ranges  continue  young  and  high.  As 

lofty  mountains, 
such  as  the  Sel- 
kirks  and  the 
Alps,  are  lowered 
by  frost  and 
glacier  ice,  the 
snowfall  will  de- 


FIG.  118.   Longitudinal  Section  of  a  Tide  Glacier  occu- 
pying a  Fjord  and  discharging  Icebergs 


Dotted  line,  sea  level 


crease,  the  line 
of  permanent 
snow  will  rise, 
and  as  the  mountain  hollows  in  which  snow  may  gather  are 
worn  beneath  the  snow  line,  the  glaciers  must  disappear.  Under 
present  climatic  conditions  the  work  of  glaciers  is  therefore  both 
local  and  of  short  duration. 

Even  the  glacial  epoch,  during  which  vast  ice  sheets  depos- 
ited drift  over  northeastern  North  America,  must  have  been 
brief  as  well  as  recent,  for  many  lofty  mountains,  such  as  the 
Ptockies  and  the  Alps,  still  bear  the  marks  of  great  glaciers 
which  then  filled  their  valleys.  Had  the  glacial  epoch  been 
long,  as  the  earth  counts  time,  these  mountains  would  have 
been  worn  low  by  ice ;  had  the  epoch  been  remote,  the  marks 
of  glaciation  would  already  have  been  largely  destroyed  by 
other  agencies. 

On  the  other  hand,  rivers  are  well-nigh  universally  at  work 
over  the  land  surfaces  of  the  globe,  and  ever  since  the  dry  land 
appeared  they  have  been  constantly  engaged  in  leveling  the 


THE  WORK  OF  GLACIERS  143 

continents  and  in  delivering  to  the  seas  the  waste  which  there 
is  huilt  into  the  stratified  rocks. 

Icebergs.  Tide  glaciers,  such  as  those  of  Greenland  and  Alaska, 
are  able  to  excavate  their  beds  to  a  considerable  distance  below 
sea  level.  From  their  fronts  the  buoyancy  of  sea  water  raises 
and  breaks  away  great  masses  of  ice  which  float  out  to  sea  as 
icebergs.  Only  about  one  seventh  of  a  mass  of  glacier  ice  floats 
above  the  surface,  and  a  berg  three  hundred  feet  high  may  be 
estimated  to  have  been  detached  from  a  glacier  not  less  than 
two  thousand  feet  thick  where  it  met  the  sea. 

Icebergs  transport  on  their  long  journeys  whatever  drift  they 
may  have  carried  when  part  of  the  glacier,  and  scatter  it,  as 
they  melt,  over  the  ocean  floor.  In  this  way  pebbles  torn  by  the 
inland  ice  from  the  rocks  of  the  interior  of  Greenland  and  gla- 
ciated during  their  carriage  in  the  ground  moraine  are  dropped 
at  last  among  the  oozes  of  the  bottom  of  the  North  Atlantic. 


CHAPTEE   VI 


THE   WORK  OF   THE   WIND 

We  are  now  to  study  the  geological  work  of  the  currents  of 
the  atmosphere,  and  to  learn  how  they  erode,  and  transport  and 
deposit  waste  as  they  sweep  over  the  land.  Illustrations  of  the 
wind's  work  are  at  hand  in  dry  weather  on  any  windy  day. 

Clouds  of  dust  are  raised 
from  the  street  and 
driven  along  by  the  gale. 
Here  the  roadway  is 
swept  bare;  and  there, 
in  sheltered  places,  the 
dust  settles  in  little 
windrows.  The  erosive 
power  of  waste-laden  cur- 
rents of  air  is  suggested 
as  the  sharp  grains  of 
flying  sand  sting  one's 
face  or  clatter  against 
the  window.  In  the 
country  one  sometimes 


FIG.  119.   A  Sandy  Region  in  a  Desert, 
the  Sahara 


Account  for  the  mounds  of  sand  on  which  the 
clumps  of  brush  are  growing 


sees  the  dust  whirled  in 
clouds  from  dry,  plowed 
fields  in  spring  and  left  in  the  lee  of  fences  in  small  drifts 
resembling  in  form  those  of  snow  in  winter. 

The  essential  conditions  for  the  wind's  conspicuous  work  are 
illustrated  in  these  simple  examples;  they  are  aridity  and  the 
absence  of  vegetation.  In  humid  climates  these  conditions  are 
only  rarely  and  locally  met ;  for  the  most  part  a  thick  growth 

144 


THE  WORK  OF  THE  WIND  145 

of  vegetation  protects  the  moist  soil  from  the  wind  with  a  cover 
of  leaves  and  stems  and  a  mattress  of  interlacing  roots.  But 
in  arid  regions  either  vegetation  is  wholly  lacking,  or  scant 
growths  are  found  huddled  in  detached  clumps,  leaving  inter- 
spaces of  unprotected  ground  (Fig.  119).  Here,  too,  the  mantle 
of  waste,  which  is  formed  chiefly  under  the  action  of  temperature 
changes,  remains  dry  and  loose  for  long  periods.  Little  or  no 
moisture  is  present  to  cause  its  particles  to  cohere,  and  they 
are  therefore  readily  lifted  and  drifted  by  the  wind. 

TRANSPORTATION  BY  THE  WIND 

In  the  desert  the  finer  waste  is  continually  swept  to  and  fro 
by  the  ever-shifting  wind.  Even  in  quiet  weather  the  air  heated 
by  contact  with  the  hot  sands  rises  in  whirls,  and  the  dust  is 
lifted  in  stately  columns,  sometimes  as  much  as  one  thousand 
feet  in  height,  which  march  slowly  across  the  plain.  In  storms 
the  sand  is  driven  along  the  ground  in  a  continuous  sheet, 
while  the  air  is  filled  with  dust.  Explorers  tell  of  sand  storms 
in  the  deserts  of  central  Asia  and  Africa,  in  which  the  air  grows 
murky  and  suffocating.  Even  at  midday  it  may  become  dark 
as  night,  and  nothing  can  be  heard  except  the  roar  of  the 
blast  and  the  whir  of  myriads  of  grains  of  sand  as  they  fly 
past  the  ear. 

Sand  storms  are  by  no  means  uncommon  in  the  arid  regions  of 
the  western  United  States.  In  a  recent  year,  six  were  reported  from 
Yuma,  Arizona.  Trains  on  transcontinental  railways  are  occasionally 
blockaded  by  drifting  sand,  and  the  dust  sifts  into  closed  passenger 
coaches,  covering  the  seats  and  floors.  After  such  a  storm  thirteen  car 
loads  of  sand  were  removed  from  the  platform  of  a  station  on  a  western 
railway. 

Dust  falls.  Dust  launched  by  upward-whirling  winds  on  the 
swift  currents  of  the  upper  air  is  often  blown  for  hundreds  of 
miles  beyond  the  arid  region  from  which  it  was  taken.  Dust 
falls  from  western  storms  are  not  unknown  even  as  far  east  as 


146 


THE  ELEMENTS  OF  GEOLOGY 


the  Great  Lakes.  In  1896  a  "black  snow"  fell  in  Chicago, 
and  in  another  dust  storm  in  the  same  decade  the  amount  of 
dust  carried  in  the  air  over  Rock  Island,  111.,  was  estimated  at 
more  than  one  thousand  tons  to  the  cubic  mile. 


FIG.  120.    A  Tract  of  Kocky  Desert,  Arabia 

By  what  process  have  these  rocks  been  broken  up  ?    Why  is  finer 
waste  here  absent  ? 

In  March,  1901,  a  cyclonic  storm  carried  vast  quantities  of  dust  from 
the  Sahara  northward  across  the  Mediterranean  to  fall  over  southern 
and  central  Europe.  On  March  8th  dust  storms  raged  in  southern 
Algeria;  two  days  later  the  dust  fell  in  Italy;  and  on  the  llth  it 
had  reached  central  Germany  and  Denmark.  It  is  estimated  that  in 
these  few  days  one  million  eight  hundred  thousand  tons  of  waste  were 
carried  from  northern  Africa  and  deposited  on  European  soil. 

We  may  see  from  these  examples  the  importance  of  the  wind 
as  an  agent  of  transportation,  and  how  vast  in  the  aggregate 
are  the  loads  which  it  carries.  There  are  striking  differences 
between  air  and  water  as  carriers  of.  waste.  Eivers  flow  in  fixed 


THE  WORK  OF  THE  WIND 


147 


and  narrow  channels  to  definite  goals.  The  channelless  streams 
of  the  air  sweep  across  broad  areas,  and,  shifting  about  continu- 
ally, carry  their  loads  back  and  forth,  now  in  one  direction  and 
now  in  another. 

WIND  DEPOSITS 

The  mantle  of  waste  of  deserts  is  rapidly  sorted  by  the  wind. 
The  coarser  rubbish,  too  heavy  to  be  lifted  into  the  air,  is  left 
to  strew  wide  tracts  with  residual  gravels  (Fig.  120).  The  sand 
derived  from  the  disintegration  of  desert  rocks  gathers  in  vast 
fields.  About  one  eighth  of  the  surface  of  the  Sahara  is  said 
to  be  thus  covered  with  drifting  sand.  In  desert  mountains,  as 
those  of  Sinai,  it  lies  like  fields  of  snow  in  the  high  valleys 
below  the  sharp  peaks.  On  more  level  tracts  it  accumulates 
in  seas  of  sand,  sometimes,  as  in  the  deserts  of  Arabia,  two 
hundred  and  more  feet  deep. 

Dunes.  The  sand  thus  accumulated  by  the  wind  is  heaped 
in  wavelike  hills  called  dunes.  In  the  desert  of  northwestern 
India,  where  the  prev- 
alent wind  is  of  great 
strength,  the  sand  is 
laid  in  longitudinal 
dunes,  i.e.  in  stripes 
running  parallel  with 
the  direction  of  the 
wind;  but  commonly 
dunes  lie,  like  ripple 
marks,  transverse  to  the 
wind  current.  On  the 
windward  side  they 


FIG.  121.   Longitudinal  Dunes,  Desert  of 
Northwestern  India 


Scale,  1  inch  =  3  miles 

show  a  long,  gentle  slope,  up  which  grains  of  sand  can  readily 
be  moved ;  while  to  the  lee  their  slope  is  frequently  as  great 
as  the  angle  of  repose  (Fig.  122).  Dunes  whose  sands  are  not 
fixed  by  vegetation  travel  slowly  with  the  wind;  for  their 


148 


THE  ELEMENTS  OF  GEOLOGY 


material  is  ever 
shifted  forward  as 
the  grains  are  driven 
up  the  windward 
slope  and,  falling 
over  the  crest,  are 
deposited  in  slanting 
layers  in  the  quiet  of 

"flip    1  PP 

FIG.  122.   A  Transverse  Dune,  Seven  Mile  Beach,    L 

New  Jersey  Like  river  deposits, 

Account  for  the  difference  of  slope  in  the  two  sides  of  wind-bio  wn    Sands 

the  dune.    Is  the  dune  marching?    In  what  direc-  are      stratified      since 
tion?    With  what  effect?    Do  the  ridges  of  the 

ripple  marks  upon  the  dune  extend  along  it  or  they  are  laid  by  CUT- 

athwart  it?    Why?  rentg   of    ^   varying 

in  intensity,  and  therefore  in  transporting  power,  which  carry 
now  finer  and  now  coarser  materials  and  lay  them,  down  where 
their  velocity  is 
checked  (Fig.  123). 
Since  the  wind  varies 
in  direction,  the 
strata  dip  in  vari- 
ous directions.  They 
also  dip  at  various 
angles,  according  to 
the  inclination  of  the 
surface  on  which 
they  were  laid. 

Dunes  occur  not 
only  in  arid  regions 
but  also  wherever 
loose  sand  lies  un- 
protected by  vegeta- 
tion from  the  wind. 
From  the  beaches  of 


FIG.  123.    Stratified  Wind-Blown  Sands, 
Bermuda  Islands 

These  islands  are  made  wholly  of  limestone,  the  prod- 
uct of  reef-building  corals,  and  of  plants  which 
also  secrete  carbonate  of  lime  from  the  sea  water. 
The  limestone  sand  of  the  beaches  has  been  blown 
up  into  great  dunes,  some  more  than  two  hundred 
feet  in  height.  Much  of  the  loose  dune  sand  has 
been  changed  to  firm  rock  by  percolating  waters, 
which  have  dissolved  some  of  the  limestone  and 
deposited  it  again  as  a  cement  between  the  grains 


THE  WORK  OF  THE  WIND 


149 


sea  and  lake  shores  the  wind  drives  inland  the  surface  sand 

left  dry  between  tides  and  after  storms,  piling  it  in  dunes  which 

may  invade  forests  and  fields  and 

bury  villages  beneath  their  slowly         — 

advancing  waves.    On  flood  plains   FIG.  124.  Cross  Section  of  Trans- 

during  summer  droughts  river  de-       ^dDune  a£ter  Revereal  °f 

posits   are  often  worked  over  bjr  JWmw    djagram     8howing   by 

the    wind ;    the    Sand    is    heaped   in          dotted  line  the  original  outline 

hummocks  and  much  of  the  fine        of  the  dune 

silt  is  caught  and  held  by  the  forests  and  grassy  fields  of  the 

bordering  hills. 

The  sand  of  shore  dunes  differs  little  in  composition  and  the 
shape  of  its  grains  from  that  of  the  beach  from  which  it  was 


FIG.  125.   Dune  Sands,  Shore  of  Lake  Michigan 

Account  for  the  dead  forest,  for  its  leaning  tree  trunks.    Is  the  lake  shore 
to  the  right  or  left?    What  has  been  the  history  of  the  landscape? 

derived.  But  in  deserts,  by  the  long  wear  of  grain  on  grain  as 
they  are  blown  hither  and  thither  by  the  wind,  all  soft  minerals 
are  ground  to  powder  and  the  sand  comes  to  consist  almost 
wholly  of  smooth  round  grains  of  hard  quartz. 


150 


THE   ELEMENTS   OF   GEOLOGY 


Some  marine  sandstones,  such  as  the  St.  Peter  sandstone  of 
the  upper  Mississippi  valley,  are  composed  so  entirely  of  polished 
spherules  of  quartz  that  it  has  been  believed  by  some  that  their 
grains  were  long  blown  about  in  ancient  deserts  before  they  were 
deposited  in  the  sea. 

Dust  deposits.  As  desert  sands  are  composed  almost  wholly 
of  quartz,  we  may  ask  what  has  become  of  the  softer  minerals 
of  which  the  rocks  whose  disintegration  has  supplied  the  sand 

were    in    part,   and 

often  in  large  part, 
composed.  The 
softer  minerals  have 
been  ground  to 
powder,  and  little 
by  little  the  quartz 
sand  also  is  worn  by 
attrition  to  fine  dust. 
Yet  dust  deposits  are 
scant  and  few  in 
great  deserts  such 
as  the  Sahara.  The 
finer  waste  is  blown 


FIG.  126.    Crescentic  Sand  Dunes,  Valley  of  the 
Columbia  River 


Did  the  wind  which  shaped  them  hlow  from  the  left 

or  from  the  right  ?  beyond  its  limits  and 

laid  in  adjacent  oceans,  where  it  adds  to  the  muds  and  oozes  of 
their  floors,  and  on  bordering  steppes  and  forest  lands,  where  it 
is  bound  fast  by  vegetation  and  slowly  accumulates  in  deposits 
of  unstratified  loose  yellow  earth.  The  fine  waste  of  the  Sahara 
has  been  identified  in  dredgings  from  the  bottom  of  the  Atlantic 
Ocean,  taken  hundreds  of  mile*  from  the  coast  of  Africa. 

Loess.  In  northern  China  an  area  as  large  as  France  is  deeply 
covered  with  a  yellow  pulverulent  earth  called  loess  (German, 
loose),  which  many  consider  a  dust  deposit  blown  from  the  great 
Mongolian  desert  lying  to  the  west.  Loess  mantles  the  recently 
uplifted  mountains  to  the  height  of  eight  thousand  feet  and 


THE  WORK  OF  THE  WIND 


151 


descends  on  the  plains  nearly  to  sea  level.  Its  texture  and  lack 
of  stratification  give  it  a  vertical  cleavage ;  hence  it  stands  in 
steep  cliffs  on  the  sides  of  the  deep  and  narrow  trenches  which 
have  been  cut  in  it  by  streams. 

On  loess  hillsides  in  China  are  thousands  of  villages  whose  cavelike 
dwellings  have  been  excavated  in  this  soft,  yet  firm,  dry  loam.  While 
dust  falls  are  common  at  the  present  time  in  this  region,  the  loess  is 
now  being  rapidly  denuded  by  streams,  and  its  yellow  silt  gives  name 
to  the  muddy  Hwang-ho  (Yellow  River),  and  to  the  Yellow  Sea,  whose 
waters  it  discolors  for  scores  of  miles  from  shore. 

Wind  deposits  both  of  dust  and  of  sand  may  be  expected  to 
contain  the  remains  of  land  shells,  bits  of  wood,  and  bones  of 
land  animals,  testifying  to  the  fact  that  they  were  accumulated 
in  open  air  and  not  hi  the  sea  or  hi  bodies  of  fresh  water. 

WIND  EROSION 


FIG.  127.  \Vind-Carved  Rocks,  Arizona 

Sand-laden  currents  of  air  abrade  and  smooth  and  polish 
exposed  rock  surfaces,  acting  in  much  the  same  way  as  does  the 


152 


THE  ELEMENTS  OF  GEOLOGY 


jet  of  steam  fed  with  sharp  sand,  which  is  used  in  the  manu- 
facture of  ground  glass.  Indeed,  in  a  single  storm  at  Cape  Cod 
a  plate  glass  of  a  lighthouse  was  so  ground  by  flying  sand  that 
its  transparency  was  destroyed  and  its  removal  made  necessary. 

Telegraph  poles  and  wires  whetted  by  wind-blown  sands  are 
destroyed  within  a  few  years.  In  rocks  of  unequal  resistance  the 

harder  parts  are  left  in  relief,  while  the 
softer  are  etched  away.  Thus  in  the  pass 
of  San  Bernardino,  CaL,  through  which 
strong  winds  stream  from  the  west,  crys- 
tals of  garnet  are  left  projecting  on  deli- 
cate rock  fingers  from  the  softer  rock  in 
which  they  were  imbedded. 

Wind-carved  pebbles  are  characteristic- 
ally planed,  the  facets  meeting  along  a 
summit  ridge  or  at  a  point  like  that  of 
a  pyramid.  We  may  suppose  that  these 
facets  were  ground  by  prevalent  winds 
from  certain  directions,  or  that  from  time 
to  time  the  stone  was  undermined  and 
rolled  over  as  the  sand  beneath  it  was 
blown  away  on  the  windward  side,  thus 
exposing  fresh  surfaces  to  the  driving 
sand.  Such  wind-carved  pebbles  are  sometimes  found  in  ancient  .rocks 
and  may  be  accepted  as  evidence  that  the  sands  of  which  the  rocks  are 
composed  were  blown  about  by  the  wind. 

Deflation.  In  the  denudation  of  an  arid  region,  wind  erosion 
is  comparatively  ineffective  as  compared  with  deflation  (Latin, 
de,  from ;  flare,  to  blow),  —  a  term  by  which  is  meant  the  con- 
stant removal  of  waste  by  the  wind,  leaving  the  rocks  bare  to 
the  continuous  attack  of  the  weather.  In  moist  climates  denu- 
dation is  continually  impeded  by  the  mantle  of  waste  and  its 
cover  of  vegetation,  and  the  land  surface  can  be  lowered  no 
faster  than  the  waste  is  removed  by  running  water.  Deep 
residual  soils  come  to  protect  all  regions  of  moderate  slope, 
concealing  from  view  the  rock  structure,  and  the  various  forms 


FIG.  128.  A  Wind-Carved 
Pebble,  Cape  Cod 


THE   WORK  OF  THE  WIND 


153 


of  the  land  are  due  more  to  the  agencies  of  erosion  and  trans- 
portation than  to  differences  in  the  resistance  of  the  under- 
lying rocks. 

But  in  arid  regions  the  mantle  is  rapidly  removed,  even  from 
well-nigh  level  plains  and  plateaus,  by  the  sweep  of  the  wind 
and  the  wash  of  occasional  rains.  The  geological  structure  of 
these  regions  of  naked  rock  can  be  read  as  far  as  the  eye  can  see, 
and  it  is  to  this  structure  that  the  forms  of  the  land  are  there 


FIG.  129.   Mesa  Verde,  Colorado 

In  the  distance  on  the  left  are  high  volcanic  mountains.  On  the  ex- 
treme right  are  seen  outliers  of  strata  which  once  covered  the  region  of 
the  mesa 

largely  due.  In  a  land  mass  of  horizontal  strata,  for  example, 
any  softer  surface  rocks  wear  down  to  some  underlying,  resist- 
ant stratum,  and  this  for  a  while  forms  the  surface  of  a  level 
plateau  (Fig.  129).  The  edges  of  the  capping  layer,  together  with 
those  of  any  softer  layers  beneath  it,  wear  back  in  steep  cliffs, 
dissected  by  the  valleys  of  wet-weather  streams  and  often  swept 
bare  to  the  base  by  the  wind.  As  they  are  little  protected 
by  talus,  which  commonly  is  removed  about  as  fast  as  formed, 
these  escarpments  and  the  walls  of  the  valleys  retreat  indefi- 
nitely, exposing  some  hard  stratum  beneath  which  forms  the 
floor  of  a  widening  terrace. 

The  high  plateaus  of  northern  Arizona  and  southern  Utah 
(Fig.  130),  north  of  the  Grand  Canyon  of  the  Colorado  Eiver,  are 


154  THE  ELEMENTS  OF  GEOLOGY 

composed  of  stratified  rocks  more  than  ten  thousand  feet  thick 
and  of  very  gentle  inclination  northward.  From  the  broad  plat- 
form in  which  the  canyon  has  been  cut  rises  a  series  of  gigantic 
stairs,  which  are  often  more  than  one  thousand  feet  high  and 
a  score  or  more  of  miles  in  breadth.  The  retreating  escarp- 
ments, the  cliff's  of  the  mesas  and  buttes  which  they  have  left 
behind  as  outliers,  and  the  walls  of  the  ravines  are  carved 
into  noble  architectural  forms  —  into  cathedrals,  pyramids, 
amphitheaters,  towers,  arches,  and  colonnades  —  by  the  processes 


EIG.  130.  North-South  Section,  Eighty-Five  Miles  Long,  across  the 
Plateau  North  of  the  Grand  Canyon  of  the  Colorado  River,  Ari- 
zona, showing  Retreating  Escarpments 

O,  outliers  ;  V,  canyon  of  the  Colorado  ;  A-H,  rock  systems  from  the 
Archaean  to  the  Tertiary ;  P,  platform  of  the  plateau  from  which  the 
once  overlying  rocks  have  been  stripped ;  dotted  lines  indicate  probable 
former  extension  of  the  strata.  How  thick  is  the  mass  of  strata  which 
has  been  removed  from  over  the  platform  ?  Has  this  work  been  accom- 
plished while  the  Colorado  River  has  been  cutting  its  present  canyon? 

of  weathering  aided  by  deflation.  It  is  thus  by  the  help  of  the 
action  of  the  wind  that  great  plateaus  in  arid  regions  are  dis- 
sected and  at  last  are  smoothed  away  to  waterless  plains,  either 
composed  of  naked  rock,  or  strewed  with  residual  gravels,  or 
covered  with  drifting  residual  sand. 

The  specific  gravity  of  air  is  ¥i¥  that  of  water.  How  does  this  fact 
affect  the  weight  of  the  material  which  each  can  carry  at  the  same 
velocity  ? 

If  the  rainfall  should  lessen  in  your  own  state  to  from  five  to  ten 
inches  a  year,  what  changes  would  take  place  in  the  vegetation  of  the 
country?  in  the  soil?  in  the  streams?  in  the  erosion  of  valleys?  in  the 
agencies  chiefly  at  work  in  denuding  the  land  ? 

In  what  way  can  a  wind-carved  pebble  be  distinguished  from  a  river- 
worn  pebble  ?  from  a  glaciated  pebble  ? 


CHAPTEE   VII 
THE   SEA  AND  ITS   SHORES 

We  have  already  seen  that  the  ocean  is  the  goal  at  which  the 
waste  of  the  land  arrives.  The  mantle  of  rock  waste,  creeping 
down  slopes,  is  washed  to  the  sea  by  streams,  together  with  the 
material  which  the  streams  have  worn  from  their  beds  and  that 
dissolved  by  underground  waters.  In  arid  regions  the  winds 


FIG.  131.   Sea  Cliff  and  Rock  Bench  Cut  in  Chalk,  Dover,  England 

sweep  waste  either  into  bordering  oceans  or  into  more  humid 
regions  where  rivers  take  it  up  and  carry  it  on  to  the  sea. 
Glaciers  deliver  the  load  of  their  moraines  either  directly  to  the 
sea  or  leave  it  for  streams  to  transport  to  the  same  goal.  All 
deposits  made  on  the  land,  such  as  the  flood  plains  of  rivers,  the 

155 


156  THE  ELEMENTS  OF  GEOLOGY 

silts  of  lake  beds,  dune  sands,  and  sheets  of  glacial  drift,  mark 
but  pauses  in  the  process  which  is  to  bring  all  the  materials  of 
the  land  now  above  sea  level  to  rest  upon  the  ocean  bed. 

But  the  sea  is  also  at  work  along  all  its  shores  as  an  agent 
of  destruction,  and  we  must  first  take  up  its  work  in  erosion 
before  we  consider  how  it  transports  and  deposits  the  waste  of 
the  land. 

SEA  EROSION 

The  sea  cliff  and  the  rock  bench.  On  many  coasts  the  land 
fronts  the  ocean  in  a  line  of  cliffs  (Fig.  131).  To  the  edge  of  the 
cliffs  there  lead  down  valleys  and  ridges,  carved  by  running 
water,  which,  if  extended,  would  meet  the  water  surface  some 
way  out  from  shore.  Evidently  they  are  now  abruptly  cut  short 

at  the  present  shore  line  because 
High  tide        ^ie  land  has  been  cut  back. 
--^,  ^lerel  Along  the  foot  of  the  cliff  lies  a 

~~^     "ently  shelving  bench  of  rock,  more 


FIG.  132.  Diagram  of  Sea  Cliff   or  less  thickly  veneered  with  sand 
ac,  and  Hock  Bench  rb  &nd  shingle>     At  low  tide  its  imier 

The    broken    line    indicates    the    marorm  is  laid  bare,  but  at  high  tide 
former  extent  of  the  land 

it  is  covered  wholly,  and  the  sea 

washes  the  base  of  the  cliffs.  A  notch,  of  which  the  sea  cliff 
and  the  rock  bench  are  the  two  sides,  has  been  cut  along  the 
shore  (Fig.  132). 

Waves.  The  position  of  the  rock  bench,  with  its  inner  margin 
slightly  above  low  tide,  shows  that  it  has  been  cut  by  some 
agent  which  acts  like  a  horizontal  saw  set  at  about  sea  level. 
This  agent  is  clearly  the  surface  agitation  of  the  water;  it  is 
the  wind-raised  wave. 

As  a  wave  comes  up  the  shelving  bench  the  crest  topples 
forward  and  the  wave  "  breaks,"  striking  a  blow  whose  force  is 
measured  by  the  momentum  of  all  its  tong  of  falling  water 
(Fig.  133).  On  the  coast  of  Scotland  the  force  of  the  blows 


THE  SEA  AND  ITS  SHORES 


157 


struck  by  the  waves  of  the  heaviest  storms  has  sometimes 
exceeded  three  tons  to  the  square  foot.  But  even  a  calm  sea 
constantly  chafes  the  shore.  It  heaves  in  gentle  undulations 
known  as  the  ground  swell,  the  result  of  storms  perhaps  a 
thousand  miles  dis- 
tant, and  breaks  on 
the  shore  in  surf. 

The  blows  of  the 
waves  are  not  struck 
with  clear  water 
only,  else  they 
would  have  little 
effect  on  cliffs  of 
solid  rock.  Storm  FlG- 133'  Breaking  Wave,  L*ke  Superior 

waves  arm  themselves  with  the  sand  and  gravel,  the  cobbles, 
and  even  the  large  bowlders  which  lie  at  the  base  of  the  cliff, 
and  beat  against  it  with  these  hammers  of  stone. 

Where  a  precipice  descends  sheer  into  deep  water,  waves  swash  up 
and  down  the  face  of  the  rocks  but  cannot  break  and  strike  effective 
blows.  They  therefore  erode  but  little  until  the  talus  fallen  from  the 
cliff  is  gradually  built  up  beneath  the  sea  to  the  level  at  which  the 
waves  drag  bottom  upon  it  and  break. 

Compare  the  ways  in  which  different  agents  abrade.  The  wind 
lightly  brushes  sand  and  dust  over  exposed  surfaces  of  rock.  Running 
water  sweeps  fragments  of  various  sizes  along  its  channels,  holding 
them  with  a  loose  hand.  Glacial  ice  grinds  the  stones  of  its  ground 
moraine  against  the  underlying  rock  with  the  pressure  of  its  enormous 
weight.  The  wave  hurls  fragments  of  rock  against  the  sea  cliff,  bruising 
and  battering  it  by  the  blow.  It  also  rasps  the  bench  as  it  drags  sand 
and  gravel  to  and  fro  upon  it. 

Weathering  of  sea  cliffs.  The  sea  cliff  furnishes  the  weapons 
for  its  own  destruction.  They  are  broken  from  it  not  only  by 
the  wave  but  also  by  the  weather.  Indeed  the  sea  cliff  weathers 
more  rapidly,  as  a  rule,  than  do  rock  ledges  inland.  It  is  abun- 
dantly wet  with  spray.  Along  its  base  the  ground  water  of  the 


158  THE  ELEMENTS  OF  GEOLOGY 

neighboring  land  finds  its  natural  outlet  in  springs  which  under- 
mine it.  Moreover,  it  is  unprotected  by  any  shield  of  talus. 
Fragments  of  rock  as  they  fall  from  its  face  are  battered  to 
pieces  by  the  waves  and  swept  out  to  sea.  The  cliff  is  thus 
left  exposed  to  the  attack  of  the  weather,  and  its  retreat  would 
be  comparatively  rapid  for  this  reason  alone. 


EIG.  134.   Sea  Caves,  La  Jolla,  California 
Copyright,  1899,  by  Detroit  Photographic  Company 

Sea  cliffs  seldom  overhang,  but  commonly,  as  in  Figure  134,  slope  sea- 
ward, showing  that  the  upper  portion  has  retreated  at  a  more  rapid 
rate  than  has  the  base.  Which  do  you  infer  is  on  the  whole  the  more 
destructive  agent,  weathering  or  the  wave? 

Draw  a  section  of  a  sea  cliff  cut  in  well  jointed  rocks  whose  joints 
dip  toward  the  land.  Draw  a  diagram  of  a  sea  cliff  where  the  joints 
dip  toward  the  sea. 

Sea  caves.  The  wave  does  not  merely  batter  the  face  of  the  cliff. 
Like  a  skillful  quarryman  it  inserts  wedges  in  all  natural  fissures,  such 
as  joints,  and  uses  explosive  forces.  As  a  wave  flaps  against  a  crevice 
it  compresses  the  air  within  with  the  sudden  stroke  ;  as  it  falls  back 
the  air  as  suddenly  expands.  On  lighthouses  heavily  barred  doors  have 
been  burst  outward  by  the  explosive  force  of  the  air  within,  as  it  was 


THE  SEA  AND  ITS  SHORES 


159 


released  from  pressure  when  a  partial  vacuum  was  formed  by  theieflu- 

ence  of  the  wave.    Where  a  crevice  is  filled  with  water  the  entire  force 

of  the  blow  of  the  wave  is  transmitted  by  hydraulic  pressure  to  the  sides 

of  the  fissure.    Thus  storm 

waves  little  by  little  pry 

and  suck   the  rock  loose, 

and  in  this  way,  and  by 

the  blows  which  they  strike 

with    the    stones    of    the 

beach,    they    quarry    out 

about  a  joint,  or  wherever 

the  rock  may  be  weak,  a 

recess  known  as  a  sea  cave, 

provided    that     the    rock 

above  is  coherent  enough 

to  form  a  roof.    Otherwise 

FIG.  135.   A  Sea  Arch,  California 


Copyright,  1899,  by  Detroit  Photographic 
Company 


an  open  chasm  results. 

Blowholes  and  sea  arches. 
As  a  sea  cave  is  drilled  back 
into  the  rock,  it  may  encounter  a  joint  or  crevice  opened  to  the  surface 
by  percolating  water.  The  shock  of  the  waves  soon  enlarges  this  to  a 
blowhole,  which  one  may  find  on  the  breezy  upland,  perhaps  a  hundred 
yards  and  more  back  from  the  cliff's  edge.  In  quiet  weather  the  blow- 
hole is  a  deep  well;  in 
storm  it  plays  a  fountain 
as  the  waves  drive  through 
the  long  tunnel  below  and 
spout  their  spray  high  in 
air  in  successive  jets.  As 
the  roof  of  the  cave  thus 
breaks  down  in  the  rear, 
there  may  remain  in  front 
for  a  while  a  sea  arch, 

similar   to   the  natural 
FIG.  136.   Chasms  worn  by  Waves,  .    . .,  .     .      , 

Coast  of  Scotland  bridSes    of    land    caverns 

(Fig.  135). 

Stacks  and  wave-cut  islands.  As  the  sea  drives  its  tunnels  and  open 
drifts  into  the  cliff,  it  breaks  through  behind  the  intervening  portions 
and  leaves  them  isolated  as  stacks,  much  as  monuments  are  detached 


100 


THE  SEA  AND  ITS  SHORES 


161 


FIG.  138.  Wave-Cut  Islands,  Scotland 
How  far  did  the  land  once  extend  ? 


from  inland  escarpments  by  the  weather ;  and  as  the  sea  cliff  retreats, 
these  remnant  masses  may  be  left  behind  as  rocky  islets.  Thus  the 
rock  bench  is  often  set  with  stacks,  islets  in  all  stages  of  destruction, 
and  sunken  reefs,  —  all 
wrecks  of  the  land  testi- 
fying to  its  retreat  before 
the  incessant  attack  of  the 
waves. 

Coves.  Where  zones 
of  soft  or  closely  jointed 
rock  outcrop  along  a 
shore,  or  where  minor 
water  courses  come 
down  to  the  sea  and  aid 
in  erosion,  the  shore  is 
worn  back  in  curved  reentrants  called  coves ;  while  the  more 
resistant  rocks  on  either  hand  are  left  projecting  as  headlands 
(Fig.  139).  After  coves  are  cut  back  a  short  distance  by  the 
waves,  the  headlands  come  to  protect  them,  as  with  break- 
waters, and  prevent  their  indefinite  retreat.  The  shore  takes  a 
curve  of  equilibrium,  along  which  the  hard  rock  of  the  exposed 

headland  and  the  weak  rock 
of  the  protected  cove  wear 
back  at  an  equal  rate. 

Rate  of  recession.  The 
rate  at  which  a  shore  recedes 
depends  on  several  factors. 
In  soft  or  incoherent  rocks 
exposed  to  violent  storms 
the  retreat  is  so  rapid  as  to 
be  easily  measured.  The  coast  of  Yorkshire,  England,  whose 
cliffs  are  cut  in  glacial  drift,  loses  seven  feet  a  year  on  the 
average,  and  since  the  Norman  conquest  a  strip  a  mile  wide, 
with  farmsteads  and  villages  and  historic  seaports,  has  been 
devoured  by  the  sea.  The  sandy  south  shore  of  Martha's 


FIG.  139.  Coves  formed  in  Softer  Strata 
S,  S  ;  while  the  Harder  Strata  H,  H, 
are  left  as  Headlands 


162 


THE  ELEMENTS  OF  GEOLOGY 


Vineyard  wears  back  three  feet  a  year.  But  hard  rocks  retreat 
so  slowly  that  their  recession  has  seldom  been  measured  by  the 
records  of  history. 

SHORE  DRIFT 

Bowlder  and  pebble  beaches.  About  as  fast  as  formed  the 
waste  of  the  sea  cliff  is  swept  both  along  the  shore  and  out  to 
sea.  The  road  of  waste  along  shore  is  the  beach.  We  may  also 
define  the  beach  as  the  exposed  edge  of  the  sheet  of  sediment 


FIG.  140.    A  Pebble  Beach,  Cape  Ann,  Massachusetts 

formed  by  the  carriage  of  land  waste  out  to  sea.  At  the  foot  of 
sea  cliffs,  where  the  waves  are  pounding  hardest,  one  commonly 
finds  the  rock  bench  strewn  on  its  inner  margin  with  large 
stones,  dislodged  by  the  waves  and  by  the  weather  and  some- 
what worn  on  their  corners  and  edges.  From  this  bowlder  beach 
the  smaller  fragments  of  waste  from  the  cliff  and  the  fragments 
into  which  the  bowlders  are  at  last  broken  drift  on  to  more  shel- 
tered places  and  there  accumulate  in  a  pebble  beach,  made  of 
pebbles  well  rounded  by  the  wear  which  they  have  suffered. 
Such  beaches  form  a  mill  whose  raw  material  is  constantly 


THE  SEA  AND  ITS  SHORES  163 

supplied  by  the  cliff.  The  breakers  of  storms  set  it  in  motion 
to  a  depth  of  several  feet,  grinding  the  pebbles  together  with 
a  clatter  to  be  heard  above  the  roar  of  the  surf.  In  such  a  rock 
crusher  the  life  of  a  pebble  is  short.  Where  ships  have  stranded 
on  our  Atlantic  coast  with  cargoes  of  hard-burned  brick  or  of  coal, 
a  year  of  time  and  a  drift  of  five  miles  along  the  shore  have  proved 
enough  to  wear  brick  and  coal  to  powder.  At  no  great  distance 
from  their  source,  therefore,  pebble  beaches  give  place  to  beaches 
of  sand,  which  occupy  the  more  sheltered  reaches  of  the  shore. 

Sand  beaches.  The  angular  sand  grains  of  various  minerals 
into  which  pebbles  are  broken  by  the  waves  are  ground  together 
under  the  beating  surf  and  rounded,  and  those  of  the  softer 
minerals  are  crushed  to  powder.  The  process,  however,  is  a 
slow  one,  and  if  we  study  these  sand  grains  under  a  lens  we 
may  be  surprised  to  see  that,  though  their  corners  and  edges 
have  been  blunted,  they  are  yet  far  from  the  spherical  form 
of  the  pebbles  from  which  they  were  derived.  The  grains  are 
small,  and  in  water  they  have  lost  about  half  their  weight  in 
air ;  the  blows  which  they  strike  one  another  are  therefore  weak. 
Besides,  each  grain  of  sand  of  the  wet  beach  is  protected  by  a 
cushion  of  water  from  the  blows  of  its  neighbors. 

The  shape  and  size  of  these  grains  and  the  relative  proportion 
of  grains  of  the  softer  minerals  which  still  remain  give  a  rough 
measure  of  the  distance  in  space  and  tune  which  they  have 
traveled  from  their  source.  The  sand  of  many  beaches,  derived 
from  the  rocks  of  adjacent  cliffs  or  brought  in  by  torrential 
streams  from  neighboring  highlands,  is  dark  with  grains  of  a 
number  of  minerals  softer  than  quartz.  The  white  sand  of  other 
beaches,  as  those  of  the  east  coast  of  Florida,  is  almost  wholly 
composed  of  quartz  grains;  for  in  its  long  travel  down  the 
Atlantic  coast  the  weaker  minerals  have  been  worn  to  powder 
and  the  hardest  alone  survive. 

How  does  the  absence  of  cleavage  in  quartz  affect  the  durability  of 
quartz  sand? 


164 


THE  ELEMENTS   OF  GEOLOGY 


How  shore  drift  migrates.  It  is  under  the  action  of  waves 
and  currents  that  shore  drift  migrates  slowly  along  a  coast. 
Where  waves  strike  a  coast  obliquely  they  drive  the  waste 
before  them  little  by  little  along  the  shore.  Thus  on  a  north- 
south  coast,  where  the  predominant  storms  are  from  the  north- 
east, there  will  be  a  migration  of  shore  drift  southwards. 

All  shores  are  swept  also  by  currents  produced  by  winds  and 
tides.  These  are  usually  far  too  gentle  to  transport  of  them- 
selves the  coarse  materials  of  which  beaches  are  made.  But 

while  the  wave  stirs 
the  grains  of  sand 
and  gravel,  and  for 
a  moment  lifts  them 
from  the  bottom, 
the  current  carries 
them  a  step  forward 
011  their  way.  The 
current  cannot  lift 
and  the  wave  can- 
not carry,  but  to- 
g  e  t  h  er  the  two 
transport  the  waste  along  the  shore.  The  road  of  shore  drift  is 
therefore  the  zone  of  the  breaking  waves. 

The  bay-head  beach.  As  the  waste  derived  from  the  wear 
of  waves  and  that  brought  in  by  streams  is  trailed  along  a 
coast  it  assumes,  under  varying  conditions,  a  number  of  dis- 
tinct forms.  When  swept  into  the  head  of  a  sheltered  bay  ft 
constitutes  the  bay-head  beach.  By  the  highest  storm  waves 
the  beach  is  often  built  higher  than  the  ground  immediately 
behind  it,  and  forms  a  dam  inclosing  a  shallow  pond  or 
marsh. 

The  bay  bar.  As  the  stream  of  shore  drift  reaches  the  mouth 
of  a  bay  of  some  size  it  often  occurs  that,  instead'  of  turning  in, 
it  sets  directly  across  toward  the  opposite  headland.  The  waste 


FIG.  141.   A  Bay  Bar,  Lake  Ontario 


THE  SEA  AND  ITS  SHORES 


165 


is  carried  out  from  shore  into  the  deeper  waters  of  the  bay 
mouth,  where  it  is  no  longer  supported  by  the  breaking  waves, 
and  sinks  to  the  bottom.  The  dump  is  gradually  built  to  the 
surface  as  a  stubby  spur,  pointing  across  the  bay,  and  as  it 
reaches  the  zone  of  wave  action  current  and  wave  can  now 
combine  to  carry  shore  drift  along  it,  depositing  their  load  con- 
tinually at  the  point  of  the  spur.  An  embankment  is  thus  con- 
structed in  much  the 
same  manner  as  a  rail- 
way fill,  which,  while 
it  is  building,  serves  as 
a  roadway  along  which 
the  dirt  from  an  ad- 
jacent cut  is  carted  to 
be  dumped  at  the  end. 

When  the  embankment  is  completed  it  bridges  the  bay  with 
a  highway  along  which  shore  drift  now  moves  without  inter- 
ruption, and  becomes  a  bay  bar. 

Incomplete  bay  bars.  Under  certain  conditions  the  sea  can- 
not carry  out  its  intention  to  bridge  a  bay.  Rivers  discharging 
in  bays  demand  open  way  to  the  ocean.  Strong  tidal  currents 
also  are  able  to  keep  open  channels  scoured  by  their  ebb  and 
flow.  In  such  cases  the  most  that  land  waste  can  do  is  to  build 
spits  .and  shoals,  narrowing  and  shoaling  the  channel  as  much 
as  possible.  Incomplete  bay  bars  sometimes  have  their  points 

recurved  by  currents  setting  at 
right  angles  to  the  stream  of  shore 
FIG.  143.  Cross  Section  of  Sand   drift,  and  are  then  classified  as 

Reef  sr,  and  Lagoon  ;  sZ,  Sea    hooks  (Fig.  142). 

Sand  reefs.    On  low  coasts 

where  shallow  water  extends  some  distance  out,  the  highway  of 
shore  drift  lies  along  a  low,  narrow  ridge,  termed  the  sand  reef, 
separated  from  the  land  by  a  narrow  stretch  of  shallow  water 
called  the  lagoon  (Fig.  143).  At  intervals  the  reef  is  held  open 


166 


THE  ELEMENTS  OF  GEOLOGY 


f 


by  inlets,  —  gaps  through  which  the  tide  flows  and  ebbs,  and  by 
which  the  water  of  streams  finds  way  to  the  sea. 

No  finer  example  of  this  kind  of  shore  line  is  to  be  found  in  the 
world  than  the  coast  of  Texas.  From  near  the  mouth  of  the  Rio 
Grande  a  continuous  sand  reef  draws  its  even 
curve  for  a  hundred  miles  to  Corpus  Christi 
Pass,  and  the  reefs  are  but  seldom  interrupted 
by  inlets  as  far  north  as  Galveston  Harbor. 
On  this  coast  the  tides  are  variable  and  ex- 
ceptionally weak,  being  less  than  one  foot  in 
height,  while  the  amount  of  waste  swept  along 
the  shore  is  large.  The  lagoon  is  extremely 
shallow,  and  much  of  it  is  a  mud  flat  too  shoal 
for  even  small  boats.  On  the  coast  of  New 
Jersey  strong  tides  are  able  to  keep  open  inlets 
at  intervals  of  from  two  to  twenty  miles  in 
spite  of  a  heavy  alongshore  drift. 


Sand  reefs  are  formed  where  the  water 
is  so  shallow  near  shore  that  storm  waves 
cannot  run  in  it  and  therefore  break  some 
distance  out  from  land.  Where  storm 
waves  first  drag  bottom  they  erode  and 
deepen  the  sea  floor,  and  sweep  in  sedi- 
ment as  far  as  the  line  where  they  break. 
Here,  where  they  lose  their  force,  they 
drop  their  load  and  beat  up  the  ridge 


FIG.  144.   Sand  Reef 
and  Lagoon,  Texas 


which  is  known  as  the  sand  reef  when  it  reaches  the  surface. 


SMOKES  OF  ELEVATION  AND  DEPRESSION 

Our  studies  have  already  brought  to  our  notice  two  distinct 
forms  of  strand  lines,  —  one  the  high,  rocky  coast  cut  back  to 
cliffs  by  the  attack  of  the  waves,  and  the  other  the  low,  sandy 
coast  where  the  waves  break  usually  upon  the  sand  reef.  To 
understand  the  origin  of  these  two  types  we  must  know  that 


THE  SEA  AND  ITS  SHORES  167 

the  meeting  place  of  sea  and  land  is  determined  primarily  by 
movements  of  the  earth's  crust.  Where  a  coast  land  emerges 
the  shore  line  moves  seaward;  where  it  is  being  submerged 
the  shore  line  advances  on  the  land. 

Shores  of  elevation.  The  retreat  of  the  sea,  either  because  of 
a  local  uplift  of  the  land  or  for  any  other  reason,  such  as  the 
lowering  of  any  portion  of  ocean  bottom,  lays  bare  the  inner 
margin  of  the  sea  floor.  Where  the  sea  floor  has  long  received 
the  waste  of  the  land  it  has  been  built  up  to  a  smooth,  subaque- 
ous plain,  gently  shelving  from  the  land.  Since  the  new  shore 
line  is  drawn  across  this  even  surface  it  is  simple  and  regular, 
and  is  bordered  on  the  one  side  by  shallow  water  gradually 
deepening  seaward,  and  on  the  other  by  low  land  composed  of 
material  which  has  not  yet  thoroughly  consolidated  to  firm  rock. 
A  sand  reef  is  soon  beaten  up  by  the  waves,  and  for  some  time 
conditions  will  favor  its  growth.  The  loss  of  sand  driven  into 
the  lagoon  beyond,  and  of  that  ground  to  powder  by  the  surf 
and  carried  out  to  sea,  is  more  than  made  up  by  the  stream  of 
alongshore  drift,  and  especially  by  the  drag  of  sediments  to  the 
reef  by  the  waves  as  they  deepen  the  sea  floor  on  its  seaward  side. 

Meanwhile  the  lagoon  gradually  fills  with  waste  from  the 
reef  and  from  the  land.  It  is  invaded  by  various  grasses  and 
reeds  which  have  learned  to  grow  in  salt  and  brackish  water; 
the  marsh,  laid  bare  only  at  low  tide,  is  built  above  high  tide 
by  wind  drift  and  vegetable  deposits,  and  becomes  a  meadow, 
soldering  the  sand  reef  to  the  mainland. 

While  the  lagoon  has  been  filling,  the  waves  have  been  so 
deepening  the  sea  floor  off  the  sand  reef  that  at  last  they  are 
able  to  attack  it  vigorously.  They  now  wear  it  back,  and,  driving 
the  shore  line  across  the  lagoon  or  meadow,  cat  a  line  of  low 
cliffs  on  the  mainland.  Such  a  shore  is  that  of  Gascony  in 
southwestern  France,  —  a  low,  straight,  sandy  shore,  bordered 
by  dunes  and  unprotected  by  reefs  from  the  attack  of  the  waves 
of  the  Bay  of  Biscay. 


168 


THE  ELEMENTS  OF  GEOLOGY 


We  may  say,  then,  that  on  shores  of  elevation  the  presence 
of  sand  reefs  and  lagoons  indicates  the  stage  of  youth,  while 

the  absence  of  these 
features  and  the 
vigorous  and  unim- 
peded attack  by  the 
sea  upon  the  mam- 
land  indicate  the 
stage  of  maturity. 
Where  much  waste 
is  brought  in  by 
rivers  the  maturity 
of  such  a  coast  may 
be  long  delayed. 
The  waste  from  the 
land  keeps  the  sea 
shallow  offshore  and 
constantly  renews 
the  sand  reef.  The 
energy  of  the  waves 
is  consumed  in  hand- 
ling shore  drift,  and 
no  energy  is  left  for 
an  effective  attack 
upon  the  land.  In- 
deed, with  an  exces- 

FIG.  145.   Map  of  New  Jersey,  with  that  Portion    sive  amount  of  waste 
of  the  State  one  Hundred  Feet  and  more  above    ,  .  , 

Sea  Level  shaded  brought   down  by 

Describe  the  coast  line  which  the  state  would  have  if    streamstheiaiia 
depressed  one  hundred  feet.     Compare  it  with  the    may  be  built  Out  and 

present  coast  line  encroach    tempo- 

rarily upon  the  sea ;  and  not  until  long  denudation  has  lowered 
the  land,  and  thus  decreased  the  amount  of  waste  from  it,  may 
the  waves  be  able  to  cut  through  the  sand  reef  and  thus  the 
coast  reach  maturity. 


THE  SEA  AND  ITS  SHORES 


169 


SHORES  OF  DEPRESSION 

Where  a  coastal  region  is  undergoing  submergence  the  shore 
line  moves  landward.  The  horizontal  plane  of  the  sea  now 
intersects  an  old  land  surface  roughened  by  subaerial  denuda- 
tion. The  shore  line  is  irregular  and  indented  in  proportion  to 
the  relief  of  the  land  and  the  amount 
of  the  submergence  which  the  land 
has  suffered.  It  follows  up  partially 
submerged  valleys,  forming  bays,  and 
bends  round  the  divides,  leaving 
them  to  project  as  promontories  and 
peninsulas.  The  outlines  of  shores 
of  depression  are  as  varied  as  are 
the  forms  of  the  land  partially  sub- 
merged. We  give  a  few  typical  illus- 
trations. 


The  characteristics  of  the  coast  of 
Maine  are  due  chiefly  to  the  fact  that  a 
mountainous  region  of  hard  rocks,  once 
worn  to  a  peneplain,  and  after  a  sub- 
sequent elevation  deeply  dissected  by 
north-south  valleys,  has  subsided,  the 
depression  amounting  on  its  southern 
margin  to  as  much  as  six  hundred  feet 
below  sea  level.  Drowned  valleys  pene- 
trate the  land  in  long,  narrow  bays,  and 
rugged  divides  project  in  long,  narrow 
land  arms  prolonged  seaward  by  islands 


FIG.  146.   Chesapeake  Bay 

Draw  a  sketch  map  of  this  area 
before  its  depression 


representing  the  high  portions  of  their  extremities.  Of  this  exceedingly 
ragged  shore  there  are  said  to  be  two  thousand  miles  from  the  Xew 
Brunswick  boundary  as  far  west  as  Portland,  —  a  straight-line  distance 
of  but  two  hundred  miles.  Since  the  time  of  its  greatest  depression 
the  land  is  known  to  have  risen  some  three  hundred  feet ;  for  the  bays 
have  been  shortened,  and  the  waste  with  which  their  floors  were  strewn 
is  now  in  part  laid  bare  as  clay  plains  about  the  bay  heads  and  in 
narrow  selvages  about  the  peninsulas  and  islands. 


170  THE   ELEMENTS  OF   GEOLOGY 

The  coast  of  Dalmatia,  on  the  Adriatic  Sea,  is  characterized  by  long 
land  arms  and  chains  of  long  and  narrow  islands,  all  parallel  to  the 
trend  of  the  coast.  A  region  of  parallel  mountain  ranges  has  been 
depressed,  and  the  longitudinal  valleys  which  lie  between  them  are 
occupied  by  arms  of  the  sea. 

Chesapeake  Bay  is  a  branching  bay  due  to  the  depression  of  an 
ancient  coastal  plain  which,  after  having  emerged  from  the  sea,  was 
channeled  with  broad,  shallow  valleys.  The  sea  has  invaded  the  valley 
of  the  trunk  stream  and  those  of  its  tributaries,  forming  a  shallow  bay 
whose  many  branches  are  all  directed  toward  its  axis  (Fig.  146). 

Hudson  Bay,  and  the  North,  the  Baltic,  and  the  Yellow  seas  are 
examples  where  the  sinking  of  the  land  has  brought  the  sea  in  over 
low  plains  of  large  extent,  thus  deeply  indenting  the  continental  out- 
line. The  rise  of  a  few  hundred  feet  would  restore  these  submerged 
plains  to  the  land. 

The  cycle  of  shores  of  depression.  In  its  infantile  stage  the 
outline  of  a  shore  of  depression  depends  almost  wholly  on  the 
previous  relief  of  the  land,  and  but  little  on  erosion  by  the  sea. 
Sea  cliffs  and  narrow  benches  appear  where  headlands  and 
outlying  islands  have  been  nipped  by  the  waves.  As  yet,  little 
shore  waste  has  been  formed.  The  coast  of  Maine  is  an  example 
of  this  stage. 

In  early  youth  all  promontories  have  been  strongly  cliffed, 
and  under  a  vigorous  attack  of  the  sea  the  shore  of  open  bays 
may  be  cut  back  also.  Sea  stacks  and  rocky  islets,  caves  and 
coves,  make  the  shore  minutely  ragged.  The  irregularity  of  the 
coast,  due  to  depression,  is  for  a  while  increased  by  differential 
wave  wear  on  harder  and  softer  rocks.  The  rock  bench  is  still 
narrow.  Shore  waste,  though  being  produced  in  large  amounts, 
is  for  the  most  part  swept  into  deeper  water  and  buried  out 
of  sight.  Examples  of  this  stage  are  the  east  coast  of  Scotland 
and  the  California  coast  near  San  Francisco. 

Later  youtli  is  characterized  by  a  large  accumulation  of 
shore  waste.  The  rock  bench  has  been  cut  back  so  that  it 
now  furnishes  a  good  roadway  for  shore  drift.  The  stream  of 


THE  SEA  AXD  ITS  SHORES 


171 


alongshore  drift  grows  larger  and  larger,  filling  the  heads  of  the 
smaller  bays  with  beaches,  building  spits  and  hooks,  and  tying 
islands  with  sand  bars  to  the  mainland.  It  bridges  the  larger 
bays  with  bay  bars,  while  their  length  is  being  reduced  as  their 
inclosing  promontories  are  cut  back  by  the  waves.  Thus  there 
comes  to  be  a  straight,  continuous,  and  easy  road,  no  longer 
interrupted  by  headlands  and  bays,  for  the  transportation  of 
waste  alongshore. 
The  Baltic  coast  of 
Germany  is  in  this 
stage. 

All  this  while 
streams  have  been 
busy  filling  with 
delta  deposits  the 
bays  into  which 
they  empty.  By 
these  steps  a  coast 
gradually  advances 
to  maturity,  the 
stage  when  the  irregularities  due  to  depression  have  been 
effaced,  when  outlying  islands  formed  by  subsidence  have  been 
planed  away,  and  when  the  shore  line  has  been  driven  back 
behind  the  former  bay  heads.  The  sea  now  attacks  the  land 
most  effectively  along  a  continuous  and  fairly  straight  line  of 
cliffs.  Although  the  first  effect  of  wave  wear  was  to  increase 
the  irregularities  of  the  shore,  it  sooner  or  later  rectifies  it, 
making  it  simple  and  smooth.  Northwestern  France  may  be 
cited  as  an  upland  plain,  dissected  and  depressed,  whose  coast 
has  reached  maturity  (Fig.  147). 

In  the  old  age  of  coasts  the  rock  bench  is  cut  back  so  far  that 
the  waves  can  no  longer  exert  their  full  effect  upon  the  shore. 
Their  energy  is  dissipated  in  moving  shore  drift  hither  and 
thither  and  in  abrading  the  bench  when  they  drag  bottom 


FIG.  147.  Portion  of  the  Northwest  Coast  of  France 


172 


THE  ELEMENTS  OF  GEOLOGY 


upon  it.  Little  by  little  the  bench  is  deepened  by  tidal  currents 
and  the  drag  of  waves ;  but  this  process  is  so  slow  that  mean- 
while the  sea  cliffs  melt  down  under  the  weather,  and  the 
bench  becomes  a  broad  shoal  where  waves  and  tides  gradually 
work  over  the  waste  from  the  land  to  greater  fineness  and  sweep 
it  out  to  sea. 

Plains   of    marine   abrasion.    While    subaerial    denudation 
reduces  the  land  to  baselevel,  the  sea  is  sawing  its  edges  to 


K 


FIG.  148.   The  South  Shore  of  Martha's  Vineyard 

The  land  is  shaded.  To  what  class  of  coasts  does  this  belong?  What  stage 
has  it  reached,  and  by  what  process  ?  What  changes  will  take  place  in 
the  future  ?  Draw  map  showing  this  coast  at  beginning  of  cycle. 

wave  base,  i.e.  the  lowest  limit  of  the  wave's  effective  wear.  The 
widened  rock  bench  forms  when  uplifted  a  plain  of  marine 
abrasion,  which  like  the  peneplain  bevels  across  strata  regardless 
of  their  various  inclinations  and  various  degrees  of  hardness. 

How  may  a  plain  of  marine  abrasion  be  expected  to  differ  from  a 
peneplain  in  its  mantle  of  waste  ? 

Compared  with  subaerial  denudation,  marine  abrasion  is  a 

comparatively  feeble  agent.    At  the  rate  of  five  feet  per  century 

—  a  higher  rate  than  obtains  on  the  youthful  rocky  coast  of 

Britain  —  it  would  require  more  than  ten  million  years  to  pare 


THE  SEA  AND  ITS  SHORES  173 

a  strip  one  hundred  miles  wide  from  the  margin  of  a  conti- 
nent, a  time  sufficient,  at  the  rate  at  which  the  Mississippi 
valley  is  now  being  worn  away,  for  subaerial  denudation  to 
lower  the  lands  of  the  globe  to  the  level  of  the  sea. 

Slow  submergence  favors  the  cutting  of  a  wide  rock  bench. 
The  water  continually  deepens  upon  the  bench;  storm  waves 
can  therefore  always  ride  in  to  the  base  of  the  cliff's  and  attack 
them  with  full  force;  shore  waste  cannot  impede  the  onset 
of  the  waves,  for  it  is  continually  washed  out  in  deeper  water 
below  wave  base. 

Basal  conglomerates.  As  the  sea  marches  across  the  land 
during  a  slow  submergence,  the  platform  is  covered  with  sheets 
of  sea-laid  sediments.  Lowest  of  these  is  a  conglomerate, — 
the  bowlder  and  pebble  beach,  widened  indefinitely  by  the 
retreat  of  the  cliffs  at  whose  base  it  was  formed,  and  preserved 
by  the  finer  deposits  laid  upon  it  in  the  constantly  deepening 
water  as  the  land  subsides.  Such  basal  conglomerates  are  not 
uncommon  among  the  ancient  rocks  of  the  land,  and  we  may 
know  them  by  their  rounded  pebbles  and  larger  stones,  com- 
posed of  the  same  kind  of  rock  as  that  of  the  abraded  and 
evened  surface  on  which  they  lie. 


CHAPTEE   VIII 
OFFSHORE   AND  DEEP-SEA   DEPOSITS 

The  alongshore  deposits  which  we  have  now  studied  are 
the  exposed  edge  of  a  vast  subaqueous  sheet  of  waste  which 
borders  the  continents  and  extends  often  for  as  much  as  two 
or  three  hundred  miles  from  land.  Soundings  show  that  off- 
shore deposits  are  laid  in  belts  parallel  to  the  coast,  the  coarsest 
materials  lying  nearest  to  the  land  and  the  finest  farthest  out. 
The  pebbles  and  gravel  and  the  clean,  coarse  sand  of  beaches 
give  place  to  broad  stretches  of  sand,  which  grows  finer  and 
finer  until  it  is  succeeded  by  sheets  of  mud.  Clearly  there  is 
an  offshore  movement  of  waste  by  which  it  is  sorted,  the 
coarser  being  sooner  dropped  and  the  finer  being  carried 
farther  out. 

OFFSHORE  DEPOSITS 

The  debris  torn  by  waves  from  rocky  shores  is  far  less  in 
amount  than  the  waste  of  the  land  brought  down  to  the  sea 
by  rivers,  being  only  one  thirty-third  as  great,  according  to  a 
conservative  estimate.  Both  mingle  alongshore  in  all  the  forms 
of  beach  and  bar  that  have  been  described,  and  both  are  together 
slowly  carried  out  to  sea.  On  the  shelving  ocean  floor  waste  is 
agitated  by  various  movements  of  the  unquiet  water,  —  by  the 
undertow  (an  outward-running  bottom  current  near  the  shore), 
by  the  ebb  and  flow  of  tides,  by  ocean  currents  where  they 
approach  the  land,  and  by  waves  and  ground  swells,  whose 
effects  are  sometimes  felt  to  a  depth  of  six  hundred  feet.  By 
all  these  means  the  waste  is  slowly  washed  to  and  fro,  and  as 
it  is  thus  ground  finer  and  finer  and  its  soluble  parts  are  more 

174 


OFFSHORE  AND  DEEP-SEA  DEPOSITS  175 

and  more  dissolved,  it  drifts  farther  and  farther  out  from  land 
It  is  by  no  steady  and  rapid  movement  that  waste  is  swept 
from  the  shore  to  its  final  resting  place.  Day  after  day  and 
century  after  century  the  grains  of  sand  and  particles  of  mud 
are  shifted  to  and  fro,  winnowed  and  spread  in  layers,  which  are 
destroyed  and  rebuilt  again  and  again  before  they  are  buried 
safe  from  further  disturbance. 

These  processes  which  are  hidden  from  the  eye  are  among 
the  most  important  of  those  with  which  our  science  has  to  do ; 
for  it  is  they  which  have  given  shape  to  by  far  the  largest  part 
of  the  stratified  rocks  of  which  the  land  is  made. 

The  continental  delta.  This  fitting  term  has  been  recently 
suggested  for  the  sheet  of  waste  slowly  accumulating  along  the 
borders  of  the  continents.  Within  a  narrow  belt,  which  rarely 
exceeds  two  or  three  hundred  miles,  except  near  the  mouths 
of  muddy  rivers  such  as  the  Amazon  and  Congo,  nearly  all  the 
waste  of  the  continent,  whether  worn  from  its  surface  by  the 
weather,  by  streams,  by  glaciers,  or  by  the  wind,  or  from  its 
edge  by  the  chafing  of  the  waves,  comes  at  last  to  its  final 
resting  place.  The  agencies  which  spread  the  material  of  the 
continental  delta  grow  more  and  more  feeble  as  they  pass  into 
deeper  and  more  quiet  water  away  from  shore.  Coarse  materials 
are  therefore  soon  dropped  along  narrow  belts  near  land.  Gravels 
and  coarse  sands  lie  in  thick,  wedge-shaped  masses  which  thin 
out  seaward  rapidly  and  give  place  to  sheets  of  finer  sand. 

Sea  muds.  Outermost  of  the  sediments  derived  from  the  waste 
of  the  continents  is  a  wide  belt  of  mud ;  for  fine  clays  settle  so 
slowly,  even  in  sea  water,  —  whose  saltness  causes  them  to  sink 
much  faster  than  they  would  in  fresh  water,  —  that  they  are 
wafted  far  before  they  reach  a  bottom  where  they  may  remain 
undisturbed.  Muds  are  also  found  near  shore,  carpeting  the 
floors  of  estuaries,  and  among  stretches  of  sandy  deposits  in 
hollows  where  the  more  quiet  water  has  permitted  the  finer 
silt  to  rest. 


176  THE  ELEMENTS  OF  GEOLOGY 

Sea  muds  are  commonly  bluish  and  consolidate  to  bluish 
shales  ;  the  red  coloring  matter  brought  from  land  waste  —  iron 
oxide  —  is  altered  to  other  iron  compounds  by  decomposing 
organic  matter  in  the  presence  of  sea  water.  Yellow  and  red 
muds  occur  where  the  amount  of  iron  oxide  in  the  silt  brought 
down  to  the  sea  by  rivers  is  too  great  to  be  reduced,  or  decom- 
posed, by  the  organic  matter  present. 

Green  muds  and  green  sand  owe  their  color  to  certain  chem- 
ical changes  which  take  place  where  waste  from  the  land  accu- 
mulates on  the  sea  floor  with  extreme  slowness.  A  greenish 

O 

mineral  called  glauconite  —  a  silicate  of  iron  and  alumina  —  is 
then  formed.  Such  deposits,  known  as  green  sand,  are  now  in 
process  of  making  in  several  patches  off  the  Atlantic  coast,  and 
are  found  on  the  coastal  plain  of  New  Jersey  among  the  off- 
shore deposits  of  earlier  geological  ages. 

Organic  deposits.  Living  creatures  swarm  along  the  shore  and 
on  the  shallows  out  from  land  as  nowhere  else  in  the  ocean. 
Seaweed  often  mantles  the  rock  of  the  sea  cliff  between  the 
levels  of  high  and  low  tide,  protecting  it  to  some  degree  from 
the  blows  of  waves.  On  the  rock  bench  each  little  pool  left 
by  the  ebbing  tide  is  an  aquarium  abounding  in  the  lowly 
forms  of  marine  life.  Below  low-tide  level  occur  beds  of  mol- 
luscous shells,  such  as  the  oyster,  with  countless  numbers  of 
other  humble  organisms.  Their  harder  parts  —  the  shells  of 
inollusks,  the  white  framework  of  corals,  the  carapaces  of  crabs 
and  other  crustaceans,  the  shells  of  sea  urchins,  the  bones  and 
teeth  of  fishes  —  are  gradually  buried  within  the  accumulating 
sheets  of  sediment,  either  whole  or,  far  more  often,  broken  into 
fragments  by  the  waves. 

By  means  of  these  organic  remains  each  layer  of  beach 
deposits  and  those  of  the  continental  delta  may  contain  a  record 
of  the  life  of  the  time  when  it  was  laid.  Such  a  record  has 
been  made  ever  since  living  creatures  with  hard  parts  appeared 
upon  the  globe.  We  shall  find  it  sealed  away  in  the  stratified 


OFFSHORE  AND  DEEP-SEA  DEPOSITS 


177 


rocks  of  the  continents,  —  parts  of  ancient  sea  deposits  now 
raised  to  form  the  dry  land.  Thus  we  have  in  the  traces  of 
living  creatures  found  in  the  rocks,  i.e.  in  fossils,  a  history  of  the 
progress  of  life  upon  the  planet. 

Molluscous  shell  deposits.  The  forms  of  marine  life  of  impor- 
tance in  rock  making  thrive  best  in  clear  water,  where  little 
sediment  is  being  laid,  and  where  at  the  same  time  the  depth  is 


FIG.  149.   Coquma,  Florida 


not  so  great  as  to  deprive  them  of  needed  light,  heat,  and  of 
sufficient  oxygen  absorbed  by  sea  water  from  the  air.  In  such 
clear  and  comparatively  shallow  water  there  often  grow  count- 
less myriads  of  animals,  such  as  mollusks  and  corals,  whose 
shells  and  skeletons  of  carbonate  of  lime  gradually  accumulate 
in  beds  of  limestone. 

A  shell  limestone  made  of  broken  fragments  cemented  together  is 
sometimes  called  coquina,  a  local  term  applied  to  such  beds  recently 
uplifted  from  the  sea  along  the  coast  of  Florida  (Fig.  149). 


178  THE  ELEMENTS  OF   GEOLOGY 

Oolitic  limestone  (oon,  an  egg  ;  lithos,  a  stone)  is  so  named  from  the 
likeness  of  the  tiny  spherules  which  compose  it  to  the  roe  of  fish. 
Corals  and  shells  have  been  pounded  by  the  waves  to  calcareous  sand, 
and  each  grain  has  been  covered  with  successive  concentric  coatings  of 
lime  carbonate  deposited  about  it  from  solution. 

The  impalpable  powder  to  which  calcareous  sand  is  ground 
by  the  waves  settles  at  some  distance  from  shore  in  deeper  and 
quieter  water  as  a  limy  silt,  and  hardens  into  a  dense,  fine- 
grained limestone  in  which  perhaps  no  trace  of  fossil  is  found 
to  suggest  the  fact  that  it  is  of  organic  origin. 

From  Florida  Keys  there  extends  south  to  the  trough  of  Florida 
Straits  a  limestone  bank  covered  by  from  five  hundred  and  forty  to 
eighteen  hundred  feet  of  water.  The  rocky  bottom  consists  of  lime- 
stone now  slowly  building  from  the  accumulation  of  the  remains  of 
mollusks,  small  corals,  sea  urchins,  worms  with  calcareous  tubes,  and 
lime-secreting  seaweed,  which  live  upon  its  surface. 

Where  sponges  and  other  silica-secreting  organisms  abound  on 
limestone  banks,  silica  forms  part  of  the  accumulated  deposit, 
either  in  its  original  condition,  as,  for  example,  the  spicules  of 
sponges,  or  gathered  into  concretions  and  layers  of  flint. 

Where  considerable  mud  is  being  deposited  along  with  car- 
bonate of  lime  there  is  in  process  of  making  a  clayey  limestone 
or  a  limy  shale ;  where  considerable  sand,  a  sandy  limestone  or 
a  limy  sandstone. 

Consolidation  of  offshore  deposits.  We  cannot  doubt  that  all 
these  loose  sediments  of  the  sea  floor  are  being  slowly  consoli- 
dated to  solid  rock.  They  are  soaked  with  water  which  carries 
in  solution  lime  carbonate  and  other  cementing  substances. 
These  cements  are  deposited  between  the  fragments  of  shells 
and  corals,  the  grains  of  sand  and  the  particles  of  mud,  binding 
them  together  into  firm  rock.  Where  sediments  have  accumu- 
lated to  great  thickness  the  lower  portions  tend  also  to  consol- 
idate under  the  weight  of  the  overlying  beds.  Except  in  the 
case  of  limestones,  recent  sea  deposits  uplifted  to  form  land  are 


179 


180 


THE  ELEMENTS   OF  GEOLOGY 


seldom  so  well  cemented  as  are  the  older  strata,  which  have 
long  been  acted  upon  by  underground  waters  deep  below  the 
surface  within  the  zone  of  cementation,  and  have- been  exposed 
to  view  by  great  erosion. 

Ripple  marks,  sun  cracks,  etc.  The  pulse  of  waves  and  tidal 
currents  agitates  the  loose  material  of  offshore  deposits,  throw- 
ing it  into  fine  parallel  ridges  called  ripple  marks.  One  may 

see  this  beautiful 
ribbing  imprinted 
on  beach  sands  un- 
covered by  the  out- 
going tide,  and  it  is 
also  produced  where 
the  water  is  of  con- 
siderable depth. 
While  the  tide  is 
out  the  surface  of 
shore  deposits  may 
be  marked  by  the 
footprints  of  birds 
and  other  animals, 
or  by  the  raindrops 
of  a  passing  shower 
(Fig.  153).  The  mud  of  flats,  thus  exposed  to  the  sun  and  dried, 
cracks  in  a  characteristic  way  (Figs.  151  and  152).  Such  mark- 
ings may  be  covered  over  with  a  thin  layer  of  sediment  at  the 
next  flood  tide  and  sealed  away  as  a  lasting  record  of  the  manner 
and  place  in  which  the  strata  were  laid.  In  Figure  150  we  have 
an  illustration  of  a  very  ancient  ripple-marked  sand  consolidated 
to  hard  stone,  uplifted  and  set  on  edge  by  movements  of  the 
earth's  crust,  and  exposed  to  open  air  after  long  erosion. 

Stratification.  For  the  most  part  the  sheet  of  sea-laid  waste 
is  hidden  from  our  sight.  Where  its  edge  is  exposed  along  the 
shore  we  may  see  the  surface  markings  which  have  just  been 


FIG.  151.    Sun  Cracks 


OFFSHORE  AND  DEEP-SEA  DEPOSITS 


181 


noticed.  Soundings  also,  and  the  observations  made  in  shallow 
waters  by  divers,  tell  something  of  its  surface;  but  to  learn 
more  of  its  structures 
we  must  study  those 
ancient  sediments  which 
have  been  lifted  from 
the  sea  and  dissected 
bysubaerial  agencies. 
From  them  we  ascertain 
that  sea  deposits  are 
stratified.  They  lie  in 
distinct  layers  which 
often  differ  from  one  an- 
other in  thickness,  in 
size  of  particles,  and 
perhaps  in  color.  They 
are  parted  by  bedding 
planes,  each  of  which 
represents  either  a 
change  in  material  or  FIG.  152.  The  Under  Side  of  a  Layer  de- 
a  pause  during  which  posited  upon  a  Sun-Cracked  Surface, 

...  showing  Casts  of  the  Cracks 

deposition    ceased    and 

the  material  of  one  layer  had  tune  to  settle  and  become  some- 
what consolidated  before  the  material  of  the  next  was  laid 
upon   it.    Stratification   is  thus    due  to  intermittently   acting 
forces,  such    as    the    agitation    of    the 
water  during  storms,  the  flow  and  ebb 
of  the  tide,  and  the   shifting   channels 
of  tidal  currents.    Off   the    mouths    of 
rivers,  stratification  is    also   caused  by 
the  coarser  and  more  abundant  material 
brought  down  at  time  of  floods  being 
laid    on    the    finer   silt   which    is    dis- 
Fio.  153.  Rain  Prints      charged  during  ordinary  stages. 


182 


THE   ELEMENTS  OF   GEOLOGY 


How  stratified  deposits  are  built  up  is  well  illustrated  in  the  flats 
which  border  estuaries,  such  as  the  Bay  of  Fundy.  Each  advance  of 
the  tide  spreads  a  film  of  mud,  which  dries  and  hardens  in  the  air 
during  low  water  before  another  film  is  laid  upon  it  by  the  next 
incoming  tidal  flood.  In  this  way  the  flats  have  been  covered  by  a  clay 
which  splits  into  leaves  as  thin  as  sheets  of  paper. 

It  is  in  fine  material,  such  as  clays  and  shales  and  limestones, 
that  the  thinnest  and  most  uniform  layers,  as  well  as  those 
of  widest  extent,  occur.  On  the  other  hand,  coarse  materials 
are  commonly  laid  in  thick  beds,  which  soon  thin  out  seaward 


FIG.  154.    Cross  Bedding  in  Sandstone,  England 


to  deposits  of 
finer  stuff.  In 
a  general  way 
strata  are  laid 
in  well-nigh 
horizontal 
sheets,  for  the 
surface  on 
which  they  are 
laid  is  generally 
of  very  gentle 

inclination.  Each  stratum,  however,  is  lenticular,  or  lenslike,  in 
form,  having  an  area  where  it  is  thickest,  and  thinning  out  thence 
to  its  edges,  where  it  is  overlapped  by  strata  similar  in  shape. 

Cross  bedding.  There  is  an  apparent  exception  to  this  rule  where 
strata  whose  upper  and  lower  surfaces  may  be  about  horizontal  are 
made  up  of  layers  inclined  at  angles  which  may  be  as  high  as  the 
angle  of  repose.  In  this  case  each  stratum  grew  by  the  addition  along 
its  edge  of  successive  layers  of  sediment,  precisely  as  does  a  sand  bar  in 
a  river,  the  sand  being  pushed  continuously  over  the  edge  and  coming 
to  rest  on  a  sloping  surface.  Shoals  built  by  strong  and  shifting  tidal 
currents  often  show  successive  strata  in  which  the  cross  bedding  is 
inclined  in  different  directions. 


OFFSHORE  AND  DEEP-SEA  DEPOSITS  183 

Thickness  of  sea  deposits.  Eemembering  the  vast  amount 
of  material  denuded  from  the  land  and  deposited  offshore,  we 
should  expect  that  with  the  lapse  of  time  sea  deposits  would 
have  grown  to  an  enormous  thickness.  It  is  a  suggestive  fact 
that,  as  a  rule,  the  profile  of  the  ocean  bed  is  that  of  a  soup 
plate,  —  a  basin  surrounded  by  a  flaring  rim.  On  the  continen- 
tal shelf,  as  the  rim  is  called,  the  water  is  seldom  more  than 
six  hundred  feet  in  depth  at  the  outer  edge,  and  shallows  grad- 
ually towards  shore.  Along  the  eastern  coast  of  the  United 
States  the  continental  shelf  is  from  fifty  to  one  hundred  and 
more  miles  in  width ;  on  the  Pacific  coast  it  is  much  narrower. 
So  far  as  it  is  due  to  upbuilding,  a  wide  continental  shelf,  such 
as  that  of  the  Atlantic  coast,  implies  a  massive  continental 
delta  thousands  of  feet  in  thickness.  The  coastal  plain  of  the 
Atlantic  states  may  be  regarded  as  the  emerged  inner  margin 
of  this  shelf,  and  borings  made  along  the  coast  probe  it  to  the 
depth  of  as  much  as  three  thousand  feet  without  finding  the 
bottom  of  ancient  offshore  deposits.  Continental  shelves  may 
also  be  due  in  part  to  a  submergence  of  the  outer  margin  of 
a  continental  plateau  and  to  marine  abrasion. 

Deposition  of  sediments  and  subsidence.  The  stratified  rocks 
of  the  land  show  in  many  places  ancient  sediments  which  reach 
a  thickness  which  is  measured  hi  miles,  and  which  are  yet  the 
product  of  well-nigh  continuous  deposition.  Such  strata  may 
prove  by  their  fossils  and  by  their  composition  and  structure 
that  they  were  all  laid  offshore  in  shallow  water.  We  must  infer 
that,  during  the  vast  length  of  time  recorded  by  the  enormous 
pile,  the  floor  of  the  sea  along  the  coast  was  slowly  sinking, 
and  that  the  trough  was  constantly  being  filled,  foot  by  foot, 
as  fast  as  it  was  depressed.  Such  gradual,  quiet  movements  of 
the  earth's  crust  not  only  modify  the  outline  of  coasts,  as 
we  have  seen,  but  are  of  far  greater  geological  importance  in 
that  they  permit  the  making  of  immense  deposits  of  stratified 
rock 


184  THE  ELEMENTS  OF   GEOLOGY 

A  slow  subsidence  continued  during  long  time  is  recorded 
also  in  the  succession  of  the  various  kinds  of  rock  that  come 
to  be  deposited  in  the  same  area,  As  the  sea  transgresses  the 
land,  i.e.  encroaches  upon  it,  any  given  part  of  the  sea  bottom 
is  brought  farther  and  farther  from  the  shore.  The  basal  con- 
glomerate formed  by  bowlder  and  pebble  beaches  comes  to  be 
covered  with  sheets  of  sand,  and  these  with  layers  of  mud  as  the 
sea  becomes  deeper  and  the  shore  more  remote ;  while  -deposits 
of  limestone  are  made  when  at  last  no  waste  is  brought  to  the 


&ea  level 


FIG.  155.   Succession  of  Deposits  recording  a  Transgressing  Sea 
c,  conglomerate ;  ss,  sandstone ;  sh,  shale ;  Im,  limestone 

place  from  the  now  distant  land,  and  the  water  is  left  clear  for 
the  growth  of  mollusks  and  other  lime-secreting  organisms. 

Rate  of  deposition.  As  deposition  in  the  sea  corresponds  to 
denudation  on  the  land,  we  are  able  to  make  a.  general  estimate 
of  the  rate  at  which  the  former  process  is  going  on.  Leaving 
out  of  account  the  soluble  matter  removed,  the  Mississippi  is 
lowering  its  basin  at  the  rate  of  one  foot  in  five  thousand  years, 
and  we  may  assume  this  as  the  average  rate  at  which  the 
earth's  land  surface  of  fifty-seven  million  square  miles  is  now 
being  denuded  by  the  removal  of  its  mechanical  waste.  But  sedi- 
ments from  the  land  are  spread  within  a  zone  but  two  or  three 
hundred  miles  in  width  along  the  margin  of  the  continents,  a 
line  one  hundred  thousand  miles  long.  As  the  area  of  deposi- 
tion—  about  twenty-five  million  square  miles  —  is  about  one 
half  the  area  of  denudation,  the  average  rate  of  deposition  must 
be  twice  the  average  rate  of  denudation,  i.e.  about  one  foot  in 
twenty-five  hundred  years.  If  some  deposits  are  made  much 
more  rapidly  than  this,  others  are  made  much  more  slowly.  If 


OFFSHORE  AND  DEEP-SEA  DEPOSITS 


185 


they  were  laid  no  faster  than  the  present  average  rate,  the  strata 
of  ancient  sea  deposits  exposed  in  a  quarry  fifty  feet  deep  repre- 
sent a  lapse  of  at  least  one  hundred  and  twenty-five  thousand 
years',  and  those  of  a  formation  five  hundred  feet  thick  required 
for  their  accumulation  one  million  two  hundred  and  fifty  thou- 
sand years. 

The  sedimentary  record  and  the  denudation  cycle.  We  have 
seen  that  the  successive  stages  in  a  cycle  of  denudation,  such  as 
that  by  which  a  land  mass 
of  lofty  mountains  is  worn 
to  low  plains,  are  marked 
each  by  its  own  peculiar 
land  forms,  and  that  the 
forms  of  the  earlier  stages 
are  more  or  less  completely 
effaced  as  the  cycle  draws 
toward  an  end.  Far  more 
lasting  records  of  each  stage 
are  left  in  the  sedimentary  deposits  of  the  continental  delta. 
Thus,  in  the  youth  of  such  a  land  mass  as  we  have  mentioned, 
torrential  streams  flowing  down  the  steep  mountain  sides  de- 
liver to  the  adjacent  sea  their  heavy  loads  of  coarse  waste,  and 
thick  offshore  deposits  of  sand  and  gravel  (Fig.  156)  record 
the  high  elevation  of  the  bordering  land.  As  the  land  is  worn 
to  lower  levels,  the  amount  and  coarseness  of  the  wraste 
brought  to  the  sea  diminishes,  until  the  sluggish  streams  carry 
only  a  fine  silt  which  settles  on  the  ocean  floor  near  to  land  in 
wide  sheets  of  mud  which  harden  into  shale.  At  last,  in  the 
old  age  of  the  region  (Fig.  157),  its  low  plains  contribute  little 
to  the  sea  except  the  soluble  elements  of  the  rocks,  and  in  the 
clear  waters  near  the  land  lime-secreting  organisms  flourish  and 
their  remains  accumulate  in  beds  of  limestone.  When  long- 
weathered  lands  mantled  with  deep,  well-oxidized  waste  are 
uplifted  by  a  gradual  movement  of  the  earth's  crust,  and  the 


FIG.  156.  Thick  Offshore  Deposits  of 
Coarse  Waste  recording  the  Presence 
of  a  Young  Mountain  Range  near 
Shore 


186  THE  ELEMENTS   OF  GEOLOGY 

mantle  is  rapidly  stripped  off  by  the  revived  streams,  the  uprise 
is  recorded  in  wide  deposits  of  red  and  yellow  clays  and  sands 
upon  the  adjacent  ocean  floor. 

Where  the  waste  brought  in  is  more  than  the  waves  can 
easily  distribute,  as  off  the  mouths  of  turbid  rivers  which  drain 
highlands  near  the  sea,  deposits  are  little  winnowed,  and  are  laid 
in  rapidly  alternating,  shaly  sandstones  and  sandy  shales. 

Where  the  highlands  are  of  igneous  rock,  such  as  granite, 
and  mechanical  disintegration  is  going  on  more  rapidly  than 
chemical  decay,  these  conditions  are  recorded  in  the  nature  of 


/Sea  level 


FIG.  157.    Offshore  Deposits  recording  the  Old  Age  of  the 
Adjacent  Land 

ss,  sandstone;  .s7i,  shale;  Im,  limestone 

the  deposits  laid  offshore.  The  waste  swept  in  by  streams  con- 
tains much  feldspar  and  other  minerals  softer  and  more  soluble 
than  quartz,  and  where  the  waves  have  little  opportunity  to 
wear  and  winnow  it,  it  comes  to  rest  in  beds  of  sandstone  in 
which  grains  of  feldspar  and  other  soft  minerals  are  abundant. 
Such  feldspathic  sandstones  are  known  as  arkose. 

On  the  other  hand,  where  the  waste  supplied  to  the  sea  comes 
chiefly  from  wide,  sandy,  coastal  plains,  there  are  deposited  off- 
shore clean  sandstones  of  well-worn  grains  of  quartz  alone.  In 
such  coastal  plains  the  waste  of  the  land  is  stored  for  ages. 
Again  and  again  they  are  abandoned  and  invaded  by  the  sea  as 
from  time  to  time  the  land  slowly  emerges  and  is  again  sub- 
merged. Their  deposits  are  long  exposed  to  the  weather,  and 
sorted  over  by  the  streams,  and  winnowed  and  worked  over 
again  and  again  by  the  waves.  In  the  course  of  long  ages  such 
deposits  thus  become  thoroughly  sorted,  and  the  grains  of  all 
minerals  softer  than  quartz  are  ground  to  mud. 


OFFSHORE  AND  DEEP-SEA  DEPOSITS  187 

DEEP-SEA  OOZES  AND  CLAYS 

Globigerina  ooze.  Beyond  the  reach  of  waste  from  the  land 
the  bottom  of  the  deep  sea  is  carpeted  for  the  most  part  with 
either  chalky  ooze  or  a  fine  red  clay.  The  surface  waters  of 
the  warm  seas  swarm  with  minute  and  lowly  animals  belong- 
ing to  the  order  of  the  Foraminif- 
era,  which  secrete  shells  of  carbonate 
of  lime.  At  death  these  tiny  white 
shells  fall  through  the  sea  water  like 
snowflakes  in  the  air,  and,  slowly  dis- 
solving, seem  to  melt  quite  away  be- 
fore they  can  reach  depths  greater 
than  about  three  miles.  Near  shore 
they  reach  bottom,  but  are  masked 
by  the  rapid  deposit  of  waste  derived 
from  the  land.  At  intermediate  FIG.  158.  Globigerina  Ooze 
i  ,T  ,1  ,i  ,1  n  under  the  Microscope 

depths  they  mantle  the  ocean  floor 

with  a  white,  soft  lime  deposit  known  as  Globigerina  ooze,  from 
a  genus  of  the  Foraminifera  which  contributes  largely  to  its 
formation. 

Red  clay.  Below  depths  of  from  fifteen  to  eighteen  thousand 
feet  the  ocean  bottom  is  sheeted  with  red  or  chocolate  colored 
clay.  It  is  the  insoluble  residue  of  seashells,  of  the  debris 
of  submarine  volcanic  eruptions,  of  volcanic  dust  wafted  by  the 
winds,  and  of  pieces  of  pumice  drifted  by  ocean  currents  far 
from  the  volcanoes  from  which  they  were  hurled.  The  red 
clay  builds  up  with  such  inconceivable  slowness  that  the  teeth 
of  sharks  and  the  hard  ear  bones  of  whales  may  be  dredged  in 
large  numbers  from  the  deep  ocean  bed,  where  they  have  lain 
unburied  for  thousands  of  years  ;  and  an  appreciable  part  of  the 
clay  is  also  formed  by  the  dust  of  meteorites  consumed  in  the 
atmosphere,  —  a  dust  which  falls  everywhere  on  sea  and  land, 
but  which  elsewhere  is  wholly  masked  by  other  deposits. 


188  THE  ELEMENTS  OF  GEOLOGY 

The  dark,  cold  abysses  of  the  ocean  are  far  less  affected  by 
change  than  any  other  portion  of  the  surface  of  the  lithosphere. 
These  vast,  silent  plains  of  ooze  lie  far  below  the  reach  of 
storms.  They  know  no  succession  of  summer  and  winter,  or  of 
night  and  day.  A  mantle  of  deep  and  quiet  water  protects  them 
from  the  agents  of  erosion  which  continually  attack,  furrow,  and 
destroy  the  surface  of  the  land.  While  the  land  is  the  area  of 
erosion,  the  sea  is  the  area  of  deposition.  The  sheets  of  sedi- 
ment which  are  slowly  spread  there  tend  to  efface  any  inequal- 
ities, and  to  form  a  smooth  and  featureless  subaqueous  plain. 

With  few  exceptions,  the  stratified  rocks  of  the  land  are 
proved  by  their  fossils  and  composition  to  have  been  laid  in 
the  sea;  but  in  the  same  way  they  are  proved  to  be  offshore, 
shallow-water  deposits,  akin  to  those  now  making  on  continen- 
tal shelves.  Deep-sea  deposits  are  absent  from  the  rocks  of  the 
land,  and  we  may  therefore  infer  that  the  deep  sea  has  never 
held  sway  where  the  continents  now  are,  —  that  the  continents 
have  ever  been,  as  now,  the  elevated  portions  of  the  lithosphere, 
and  that  the  deep  seas  of  the  present  have  ever  been  its  most 
depressed  portions. 

THE  KEEF-BUILDING  CORALS 

In  warm  seas  the  most  conspicuous  of  rock-making  organisms 
are  the  corals  known  as  the  reef  builders.  Floating  in  a  boat 
over  a  coral  reef,  as,  for  example,  off  the  south  coast  of  Florida 
or  among  the  Bahamas,  one  looks  down  through  clear  water  on 
thickets  of  branching  coral  shrubs  perhaps  as  much  as  eight 
feet  high,  and  hemispherical  masses  three  or  four  feet  thick,  all 
abloom  with  countless  minute  flowerlike  coral  polyps,  gorgeous 
in  their  colors  of  yellow,  orange,  green,  and  red.  In  structure 
each  tiny  polyp  is  little  more  than  a  fleshy  sac  whose  mouth 
is  surrounded  with  petal-like  tentacles,  or  feelers.  From  the 
sea  water  the  polyps  secrete  calcium  carbonate  and  build  it  up 
into  the  stony  framework  which  supports  their  colonies.  Boring 


OFFSHORE  AND  DEEP-SEA  DEPOSITS  189 

mollusks,  worms,  and  sponges  perforate  and  honeycomb  this 
framework  even  while  its  surface  is  covered  with  myriads  of 
living  polyps.  It  is  thus  easily  broken  by  the  waves,  and  white 
fragments  of  coral  trees  strew  the  ground  beneath.  Brilliantly 
colored  fishes  live  in  these  coral  groves,  and  countless  mollusks, 
sea  urchins,  and  other  forms  of  marine  life  make  here  their 


FIG.  159.   Patch  of  Growing  Corals  exposed  at  an  Exceptionally  Low 
Tide,  Great  Barrier  Reef,  Australia 

home.  With  the  debris  from  all  these  sources  the  reef  is  con- 
stantly built  up  until  it  rises  to  low-tide  level.  Higher  than  this 
the  corals  cannot  grow,  since  they  are  killed  by  a  few  hours' 
exposure  to  the  air. 

When  the  reef  has  risen  to  wave  base,  the  waves  abrade  it 
on  the  windward  side  and  pile  to  leeward  coral  blocks  torn 
from  their  foundation,  filling  the  interstices  with  finer  fragments. 
Thus  they  heap  up  along  the  reef  low,  narrow  islands  (Fig.  160). 


190  THE   ELEMENTS  OF  GEOLOGY 

Keef  building  is  a  comparatively  rapid  progress.  It  has  been 
estimated  that  off  Florida  a  reef  could  be  built  up  to  the  surface 
from  a  depth  of  fifty  feet  in  about  fifteen  hundred  years. 

Coral  limestones.    Limestones  of  various  kinds  are  due  to  the 

reef  builders.    The  reef  rock  is  made  of  corals  in  place  and 

broken  fragments  of  all  sizes,  cemented  together  with  calcium 

carbonate  from  solution  by  infiltrating  waters.    On  the  island 

beaches   coral  sand  is  forming  oolitic 

r'^r^^^^f^^         limestone,  and   the   white   coral   mud 

with  which  the  sea  is  milky  for  miles 
FIG.  160.  Wave-Built  Island  . 

on  Coral  Reef  about  the  reef  in  times  of  storm  settles 

and  concretes  into  a  compact  limestone 

r,  reef ;  s,  sea  level 

of  finest  grain.  Corals  have  been 

among  the  most  important  limestone  builders  of  the  sea  ever 
since  they  made  their  appearance  in  the  early  geological  ages. 

The  areas  on  which  coral  limestone  is  now  forming  are  large. 
The  Great  Barrier  Reef  of  Australia,  which  lies  off  the  north- 
eastern coast,  is  twelve  hundred  and  fifty  miles  long,  and  has  a 
width  of  from  ten  to  ninety  miles.  Most  of  the  islands  of  the 
tropics  are  either  skirted  with  coral  reefs  or  are  themselves  of 
coral  formation. 

Conditions  of  coral  growth.  Reef-building  corals  cannot  live 
except  in  clear  salt  water  less,  as  a  rule,  than  one  hundred  and 
fifty  feet  in  depth,  with  a  winter  temperature  not  lower  than 
68°  F.  An  important  condition  also  is  an  abundant  food  sup- 
ply, and  this  is  best  secured  in  the  path  of  the  warm  oceanic 
currents. 

Coral  reefs  may  be  grouped  in  three  classes,  —  fringing  reefs, 
barrier  reefs,  and  atolls. 

Fringing  reefs.  These  take  their  name  from  the  fact  that  they  are 
attached  as  narrow  fringes  to  the  shore.  An  example  is  the  reef  which 
forms  a  selvage  about  a  mile  wide  along  the  northeastern  coast  of 
Cuba.  The  outer  margin,  indicated  by  the  line  of  white  surf,  where 
the  corals  are  in  vigorous  growth,  rises  from  about  forty  feet  of  water. 


OFFSHORE  AND  DEEP-SEA  DEPOSITS  191 

Between  this  and  the  shore  lies  a  stretch  of  shoal  across  which  one  can 
wade  at  low  water,  composed  of  coral  sand  with  here  and  there  a  clump 
of  growing  coral. 

Barrier  reefs.  Eeefs  separated  from  the  shore  by  a  ship 
channel  of  quiet  water,  often  several  miles  in  width  and  some- 
times as  much  as  three  hundred  feet  in  depth,  are  known  as 
barrier  reefs.  The  seaward  face  rises  abruptly  from  water  too 
deep  for  coral  growth.  Low  islands  are  cast  up  by  the  waves 
upon  the  reef,  and  inlets  give  place  for  the  ebb  and  flow  of  the 
tides.  Along  the  west  coast  of  the  island  of  New  Caledonia 
a  barrier  reef  extends  for  four  hundred  miles,  and  for  a  length 
of  many  leagues  seldom  approaches  within  eight  miles  of  the 
shore. 

Atolls.  These  are  ring-shaped  or  irregular  coral  islands,  or 
island-studded  reefs,  inclosing  a  central  lagoon.  The  narrow 
zone  of  land,  like  the  rim  of  a  great  bowl  sunken  to  the  water's 
edge,  rises  hardly  more  than  twenty  feet  at  most  above  the  sea, 
and  is  covered  with  a  forest  of  trees  such  as  the  cocoanut,  whose 
seeds  can  be  drifted  to  it  uninjured  from  long  distances.  The 
white  beach  of  coral  sand  leads  down  to  the  growing  reef,  on 
whose  outer  margin  the  surf  is  constantly  breaking.  The  sea 
face  of  the  reef  falls  off  abruptly,  often  to  depths  of  thousands 
of  feet,  while  the  lagoon  varies  in  depth  from  a  few  feet  to  one 
hundred  and  fifty  or  two  hundred,  and  exceptionally  measures 
as  much  as  three  hundred  and  fifty  feet. 

Theories  of  coral  reefs.  Fringing  reefs  require  no  explanation, 
since  the  depth  of  water  about  them  is  not  greater  than  that  at 
which  coral  can  grow ;  but  barrier  reefs  and  atolls,  which  may 
rise  from  depths  too  great  for  coral  growth  demand  a  theory  of 
their  origin. 

Darwin's  theory  holds  that  barrier  reefs  and  atolls  are  formed 
from  fringing  reefs  by  subsidence.  The  rate  of  sinking  cannot 
be  greater  than  that  of  the  upbuilding  of  the  reef,  since  other- 
wise the  corals  would  be  carried  below  their  depth  and  drowned. 


192  THE  ELEMENTS   OF  GEOLOGY 

The  process  is  illustrated  in  Figure  161,  where  v  represents  a  vol- 
canic island  in  mid  ocean  undergoing  slow  depression,  and  ss  the 
sea  level  before  the  sinking  began,  when  the  island  was  surrounded 
by  a  fringing  reef.  As  the  island  slowly  sinks,  the  reef  builds 
up  with  equal  pace.  It  rears  its  seaward  face  more  steep  than 
the  island  slope,  and  thus  the  intervening  space  between  the  sink- 
ing, narrowing  land  and  the  outer  margin  of  the  reef  constantly 
widens.  In  this  intervening  space  the  corals  are  more  or  less 
smothered  with  silt  from  the  outer  reef  and  from  the  land,  and 
are  also  deprived  in  large  measure  of  the  needful  supply  of  food 


FIG.  161.   Diagram  illustrating  the  Subsidence  Theory  of  Coral  Reefs 

and  oxygen  by  the  vigorous  growth  of  the  corals  on  the  outer 
rim.  The  outer  rim  thus  becomes  a  barrier  reef  and  the  inner 
belt  of  retarded  growth  is  deepened  by  subsidence  to  a  ship  chan- 
nel, sfs'  representing  sea  level  at  this  time.  The  final  stage, 
where  the  island  has  been  carried  completely  beneath  the  sea 
and  overgrown  by  the  contracting  reef,  whose  outer  ring  now 
forms  an  atoll,  is  represented  by  sffs". 

In  several  instances,  however,  atolls  and  barrier  reefs  may 
be  explained  without  subsidence.  Thus  a  barrier  reef  may  be 
formed  by  the  seaward  growtli  of  a  fringing  reef  upon  the  talus 
of  its  sea  face.  In  Figure  162  /  is  a  fringing  reef  whose  outer 
wall  rises  from  about  one  hundred  and  fifty  feet,  the  lower  limit 
of  the  reef-building  species.  At  the  foot  of  this  submarine  cliff 
a  talus  of  fallen  blocks  t  accumulates,  and  as  it  reaches  the  zone 


OFFSHORE  AND  DEEP-SEA  DEPOSITS 


193 


FIG.  162.   Barrier  Reef  formed 
without  Subsidence 

,  zone  of  coral  growth;  /,  former 
fringing  reef ;  t,  talus;  6, .barrier 
reef 


of  coral  growth  becomes  the  foundation  on  which  the  reef  is 

steadily  extended  seaward.    As  the  reef  widens,  the  polyps  of 

the  circumference  nourish,  while  those  of  the  inner  belt  are 

retarded  in  their  growth  and  at 

last  perish.    The  coral  rock  of  the 

inner  belt  is  now  dissolved  by  sea 

water  and  scoured  out  by  tidal 

currents  until  it  gives  place  to  a 

gradually  deepening  ship  channel, 

while  the  outer  margin  is  left  as 

a  barrier  reef. 

In  much  the  same  way  atolls 
may  be  built  on  any  shoal  which  lies  within  the  zone  of  coral 
growth.  Such  shoals  may  be  produced  when  volcanic  islands 
are  leveled  by  waves  and  ocean  currents,  and  when  subma- 
rine plateaus,  ridges,  and  peaks  are  built  up  by  various  organic 
agencies,  such  as  molluscous  and  foraminiferal  shell  deposits 
(Fig.  163).  The  reef-building  corals,  whose  eggs  are  drifted 

widely  over  the  tropic  seas 
by  ocean  currents,  colonize 
such  submarine  foundations 
wherever  the  conditions  are 
favorable  for  their  growth. 
As  the  reef  approaches  the 
FIG.  163.  Section  of  Atoll  on  a  Shoal    surface  the  corals  of  the  in- 
which  has  been  built  up  to  near  the     ner  area  are   smothered   by 


Surface  by  Organic  Deposits  upon  a 
Submarine  Volcanic  Peak 

v,  volcano;  /,  foraminiferal  deposits;    m, 


silt  and  starved,  and  their 
hard  parts  are  dissolved  and 

molluscous 'shell  deposits;  c,  coral  reef;      SCOUred   a  way  ;    while  those 

of  the  circumference,  with 

abundant  food  supply,  nourish  and  build  the  ring  of  the  atoll. 
Atolls  may  be  produced  also  by  the  backward  drift  of  sand  from 
either  end  of  a  crescentic  coral  reef  or  islard,  the  spits  uniting 
in  the  quiet  water  of  the  lee  to  inclose  a  lagoon.  In  the  Maldive 


194         THE  ELEMENTS  OF  GEOLOGY 

Archipelago  all  gradations  between  crescent-shaped  islets  and 
complete  atoll  rings  have  been  observed. 

Barrier  and  fringing  reefs  are  commonly  interrupted  off  the  mouths 
of  rivers.  Why  ? 

In  many  volcanic  islands  surrounded  with  barrier  reefs  the  shores  of 
the  islands  are  indented  with  wedge-shaped  bays  separated  by  tapering 
spurs.  Which  theory  do  such  embayed  coasts  support  ? 

On  Funafuti  atoll  a  boring  was  sunk  more  than  a  thousand  feet 
in  lime  carbonate  rock.  What  inference  may  be  drawn  as  to  the  origin 
of  this  atoll  ? 

Christmas  island,  in  the  Indian  ocean,  which  rises  eleven  hundred 
feet  above  sea  level,  is  an  old  volcanic  pile.  The  summit  of  the  volcano 
is  covered  with  thick  beds  of  limestone  made  up  of  f oraminiferal  remains. 
Upon  this  limestone  rests  a  rim  of  hills  of  coral  rock  which  represent 
the  ring  of  islets  of  an  old  atoll.  Give  the  history  of  the  island. 

Summary.  We  have  seen  that  the  ocean  bed  is  the  goal  to 
which  the  waste  of  the  rocks  of  the  land  at  last  arrives.  Their 
soluble  parts,  dissolved  by  underground  waters  and  carried  to 
the  sea  by  rivers,  are  largely  built  up  by  living  creatures  into 
vast  sheets  of  limestone.  The  less  soluble  portions  —  the  waste 
brought  in  by  streams  and  the  waste  of  the  shore  —  form  the 
muds  and  sands  of  continental  deltas.  All  of  these  sea  deposits 
consolidate  and  harden,  and  the  coherent  rocks  of  the  land  are 
thus  reconstructed  on  the  ocean  floor.  But  the  destination  is 
not  a  final  one.  The  stratified  rocks  of  the  land  are  for  the 
most  part  ancient  deposits  of  the  sea,  which  have  been  lifted 
above  sea  level;  and  we  may  believe  that  the  sediments  now 
being  laid  offshore  are  the  "  dust  of  continents  to  be,"  and  will 
sometime  emerge  to  form  additions  to  the  land.  We  are  now  to 
study  the  movements  of  the  earth's  crust  which  restore  the  sedi- 
ments of  the  sea  to  the  light  of  day,  and  to  whose  beneficence 
we  owe  the  habitable  lands  of  the  present. 


PART  II 

.    INTERNAL   GEOLOGICAL  AGENCIES 

CHAPTEE  IX 
MOVEMENTS   OF  THE   EARTH'S  CRUST 

The  geological  agencies  which  we  have  so  far  studied  — 
weathering,  streams,  underground  waters,  glaciers,  winds,  and 
the  ocean  —  all  work  upon  the  earth  f rorn  without,  and  all  are 
set  in  motion  by  an  energy  external  to  the  earth,  namely,  the 
radiant  energy  of  the  sun.  All,  too,  have  a  common  tendency 
to  reduce  the  inequalities  of  the  earth's  surface  by  leveling  the 
lands  and  strewing  their  waste  beneath  the  sea. 

But  despite  the  unceasing  efforts  of  these  external  agencies, 
they  have  not  destroyed  the  continents,  which  still  rear  their 
broad  plains  and  great  plateaus  and  mountain  ranges  above  the 
sea.  Either,  then,  the  earth  is  very  young  and  the  agents  of 
denudation  have  not  yet  had  time  to  do  their  work,  or  they 
have  been  opposed  successfully  by  other  forces. 

We  enter  now  upon  a  department  of  our  science  which  treats 
of  forces  which  work  upon  the  earth  from  within,  and  increase 
the  inequalities  of  its  surface.  It  is  they  which  uplift  and  re- 
create the  lands  which  the  agents  of  denudation  are  continually 
destroying;  it  is  they  which  deepen  the  ocean  bed  and  thus 
withdraw  its  waters  from  the  shores.  At  times  also  these  forces 
have  aided  in  the  destruction  of  the  lands  by  gradually  lower- 
ing them  and  bringing  in  the  sea.  Under  the  action  of  forces 
resident  within  the  earth  the  crust  slowly  rises  or  sinks ;  from 

196 


196  THE  ELEMENTS  OF  GEOLOGY 

time  to  time  it  has  been  folded  and  broken ;  while  vast  quanti- 
ties of  molten  rock  have  been  pressed  up  into  it  from  beneath 
and  outpoured  upon  its  surface.  We  shall  take  up  these  phe- 
nomena in  the  following  chapters,  which  treat  of  upheavals 
and  depressions  of  the  crust,  foldings  and  fractures  of  the  crust, 
earthquakes,  volcanoes,  the  interior  conditions  of  the  earth, 
mineral  veins,  and  metamorphism. 

OSCILLATIONS  OF  THE  CRUST 

Of  the  various  movements  of  the  crust  due  to  internal  agen- 
cies we  will  consider  first  those  called  oscillations,  which  lift  or 
depress  large  areas  so  slowly  that  a  long  time  is  needed  to  pro- 
duce perceptible  changes  of  level,  and  which  leave  the  strata  in 
nearly  their  original  horizontal  attitude.  These  movements  are 
most  conspicuous  along  coasts,  where  they  can  be  referred  to 
the  datum  plane  of  sea  level.  Slow  and  tranquil  oscillations  are 
recorded  along  many  shores.  Some  shores  are  emerging  from  the 
sea ;  some  are  being  submerged  by  it ;  and  no  part  of  the  land 
seems  to  have  been  exempt  from  such  changes  in  the  past. 

In  the  use  of  sea  level  as  a  datum  plane,  allowance  must  be  made  for 
the  fact  that  this  level  is  neither  unchangeable  nor  everywhere  the  same. 

Considering  the  gravitative  attraction  of  large  mountain  masses,  what 
would  you  infer  as  to  differences  in  distance  from  the  earth's  center  of 
sea  level  at  San  Francisco  and  New  Orleans?  at  Calcutta  and  Ceylon? 

What  general  and  local  effects  on  sea  level  and  shore  lines  would  be 
produced  by 

1.  The  melting  of  the  inland  ice  of  Greenland? 

2.  The  accumulation  of  ice  sheets,  such  as  those  of  the  glacial  epoch, 

over  Europe  and  Xorth  America? 

3.  A  renewed  uplift  of  the  Appalachians  raising  them  to  Alpine 

height?  • 

4.  The  filling  of  the  Gulf  of  Mexico  with  river  silts  ? 

5.  A  downwarp  of  the  floor  of   the  Gulf  of  Mexico  doubling  the 

capacity  of  the  basin? 
0.  A  more  and  a  less  rapid  rotation  of  the  earth  ? 


MOVEMENTS   OF  THE  EARTH'S  CRUST  197 

Evidences  of  changes  of  level.  Taking  the  surface  of  the 
sea  as  a  level  of  reference,  we  may  accept  as  proofs  of  relative 
upheaval  whatever  is  now  found  in  place  above  sea  level  and 
could  have  been  formed  only  at  or  beneath  it,  and  as  proofs  of 
relative  subsidence  whatever  is  now  found  beneath  the  sea  and 
could  only  have  been  formed  above  it. 

Thus  old  strand  lines  with  sea  cliffs,  wave-cut  rock  benches, 
and  beaches  of  wave-worn  pebbles  or  sand,  are  striking  proofs 
of  recent  emergence  to  the  amount  of  their  present  height  above 
tide.  No  less  conclusive  is  the  presence  of  sea-laid  rocks  which 
we  may  find  in  the  neighboring  quarry  or  outcrop,  although  it 
may  have  been  long  ages  since  they  were  lifted  from  the  sea  to 
form  part  of  the  dry  land. 

Among  common  proofs  of  subsidence  are  roads  and  buildings 
and  other  works  of  man,  and  vegetal  growths  and  deposits,  such 
as  forest  grounds  and  peat  beds,  now  submerged  beneath  the  sea. 
In  the  deltas  of  many  large  rivers,  such  as  the  Po,  the  Nile,  the 
Ganges,  and  the  Mississippi,  buried  soils  prove  subsidences  of 
hundreds  of  feet;  and  in  several  cases,  as  in  the  Mississippi 
delta,  the  depression  seems  to  be  now  in  progress. 

Other  proofs  of  the  same  movement  are  drowned  land  forms 
which  are  modeled  only  in  open  air.  Since  rivers  cannot  cut 
their  valleys  farther  below  the  baselevel  of  the  sea  than  the 
depths  of  their  channels,  drowned  valleys  are  among  the  plainest 
proofs  of  depression.  To  this  class  belong  Narragansett,  Dela- 
ware, Chesapeake,  Mobile,  and  San  Francisco  bays,  and  many 
other  similar  drowned  valleys  along  the  coasts  of  the  United 
States.  Less  conspicuous  are  the  submarine  channels  which, 
as  soundings  show,  extend  from  the  mouths  of  a  number  of 
rivers  some  distance  out  to  sea.  Such  is  the  submerged  chan- 
nel which  reaches  from  New  York  Bay  southeast  to  the  edge 
of  the  continental  shelf,  and  which  is  supposed  to  have  been 
cut  by  the  Hudson  Eiver  when  this  part  of  the  shelf  was  a 
coastal  plain. 


198         THE  ELEMENTS  OF  GEOLOGY 

Warping.  In  a  region  undergoing  changes  of  level  the  rate 
of  movement  commonly  varies  in  different  parts.  Portions  of 
an  area  may  be  rising  or  sinking  while  adjacent  portions  are 
stationary  or  moving  in  the  opposite  direction.  In  this  way  a 
land  surface  becomes  warped.  Thus,  the  eastern  end  of  the 
island  of  Crete  is  sinking,  for  ruins  of  ancient  buildings  are 
now  seen  off  shore  beneath  the  water ;  while  the  west  and  south 
coasts  are  rising,  as  is  proved  by  old  docks  which  now  stand 
twenty-seven  feet  above  sea  level. 

Since  the  close  of  the  glacial  epoch  the  coasts  of  Newfoundland 
and  Labrador  have  risen  hundreds  of  feet,  but  the  rate  of  emergence 
vo  ^  has  not  been  uniform.  The  old 

<3    strand    line,    which    stands   at 

|  ;    five   hundred   and   seventy-five 

f  %    feet  above  tide  at  St.  John's, 

Newfoundland,  declines  to  two 
hundred    and    fifty    feet    near 
sea  Level         ^Q  northern  point  of  Labrador 
FIG.  164.     "NVarped  Strand  Line  from      (Fig.  164). 

St.  John's,  Newfoundland,  to  Nach-          Lake  shores.    In  the  interior 

vak  Labrador  p         .- 

oi  continents  warpings  are  reg- 
istered by  lakes.  Any  canting  of  the  basin  of  a  lake  causes  the  water 
to  rise  on  one  side  and  to  withdraw  upon  the  other.  The  old  strand  line 
is  left  tilted  according  to  the  measure  of  the  canting. 

Thus,  the  strand  line  of  Lake  Bonneville  (p.  107)  is  no  longer  hori- 
zontal, but  in  some  parts  stands  three  hundred  and  fifty  feet  higher  than 
in  others.  The  basins  of  the  Karst  (p.  47)  show  old  terraces  which  de- 
cline toward  the  southwest.  One  of  these  basins  nearest  to  the  sea,  that 
of  Scutari,  filled  with  fresh  ground  water,  is  depressed  so  that  its  bottom 
is  one  hundred  and  twenty-four  feet  below  sea  level.  Compare  this 
warping  with  the  character  of  the  Dalmatian  coast  (p.  170). 

The  shores  of  the  Great  Lakes  show  evidences  of  warping  now  in 
progress.  On  the  southwest  shore  of  Lake  Superior  the  rising  water  is 
invading  forests  and  encroaching  upon  roads.  It  has  converted  the 
mouths  of  rivers  into  estuaries.  Advancing  up  valleys,  it  has  effaced 
rapids  within  historic  times.  On  the  opposite  side  of  the  lake  the  en- 
tering streams  are  swift  and  shallow. 


MOVEMENTS  OF  THE  EARTH'S  CRUST      199 

At  the  western  end  of  Lake  Erie  are  found  submerged  caves  contain- 
ing stalactites,  and  old  meadows  and  forest  grounds  are  now  under 
water.  In  Sandusky  Bay  the  water  is  rising  at  the  rate  of  about  two 
feet  per  century. 

The  ancient  beaches  of  the  expanded  lakes  which  occupied  the  basins 
of  the  Great  Lakes  at  the  close  of  the  glacial  epoch  decline  to  the  south 
and  west  and  show  a  long-continued  rise  of  the  land  to  the  northeast. 

Physiographic  effects  of  oscillations.  We  have  already  men- 
tioned several  of  the  most  important  effects  of  movements  of 
elevation  and  depression,  such  as  their  effects  on  rivers,  the 
mantle  of  waste  (pp.  85,  86),  and  the  forms  of  coasts  (p.  166). 
Movements  of  elevation  —  including  uplifts  by  folding  and 
fracture  of  the  crust  to  be  noticed  later  —  are  the  necessary 
conditions  for  erosion  by  whatever  agent.  They  determine  the 
various  agencies  which  are  to  be  chiefly  concerned  in  the  wear 
of  any  land,  —  whether  streams  or  glaciers,  weathering  or  the 
wind,  —  and  the  degree  of  their  efficiency.  The  lands  must 
be  uplifted  before  they  can  be  eroded,  and  since  they  must  be 
eroded  before  their  waste  can  be  deposited,  movements  of  ele- 
vation are  a  prerequisite  condition  for  sedimentation  also.  Sub- 
sidence is  a  necessary  condition  for  deposits  of  great  thickness, 
such  as  those  of  the  Great  Valley  of  California  and  the  Indo- 
Gangetic  plain  (p.  101),  the  Mississippi  delta  (p.  109),  and  the 
still  more  important  formations  of  the  continental  delta  in 
gradually  sinking  troughs  (p.  183).  It  is  not  too  much  to  say 
that  the  character  and  thickness  of  each  formation  of  the  strati- 
fied rocks  depend  primarily  on  these  crustal  movements. 

Along  the  Baltic  coast  of  Sweden,  bench  marks  show  that  the  sea  is 
withdrawing  from  the  land  at  a  rate  which  at  the  north  amounts  to 
between  three  and  four  feet  per  century.  Towards  the  south  the  rate 
decreases.  South  of  Stockholm,  until  recent  years,  the  sea  has  gained 
upon  the  land,  and  here  in  several  seaboard  towns  streets  by  the  shore 
are  still  submerged.  The  rate  of  oscillation  increases  also  from  the  coast 
inland.  On  the  other  hand,  along  the  German  coast  of  the  Baltic  the 
only  historic  fluctuations  of  sea  level  are  those  which  may  be  accounted 


200 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


201 


for  by  variations  due  to  changes  in  rainfall.  In  1730  Celsius  explained 
the  changes  of  level  of  the  Swedish  coast  as  due  to  a  lowering  of  the 
Baltic  instead  of  to  an  elevation  of  the  land.  Are  the  facts  just  stated 
consistent  with  his  theory? 

At  the  little  town  of  Tadousac  —  where  the  Saguenay  River  emp- 
ties into  the  St.  Lawrence  —  there  are  terraces  of  old  sea  beaches,  some 
almost  as  fresh  as  recent  railway  fills,  the  highest  standing  two  hundred 
and  thirty  feet  above  the  river 
(Fig.  165).  Here  the  Saguenay 
is  eight  hundred  and  forty  feet 
in  depth,  and  the  tide  ebbs 
and  flows  far  up  its  stream. 
Was  its  channel  cut  to  this 


"- 


FIG.  166.   Diagram  showing  Ruins  of 
Temple,  North  of  Naples 


C,  ancient  sea  cliff ;  m,  marble  pillars, 
dotted  where  bored  by  mollusks;  si, 
present  sea  level 


depth  by  the  river  when  the 
land  was. at  its  present  height? 
What  oscillations  are  here  re- 
corded, and  to  what  amount  ? 

A  few  miles  north  of  Naples,  Italy,  the  ruins  of  an  ancient  Roman 
temple  lie  by  the  edge  of  the  sea,  on  a  narrow  plain  which  is  overlooked 
in  the  rear  by  an  old  sea  cliff  (Fig.  166).  Three  marble  pillars  are  still 
standing.  For  eleven  feet  above  their  bases  these  columns  are  unin- 
jured, for  to  this  height  they  were  protected  by  an  accumulation  of 
volcanic  ashes ;  but  from  eleven  to  nineteen  feet  they  are  closely  pitted 
with  the  holes  of  boring  marine  mollusks.  From  these  facts  trace  the 
history  of  the  oscillations  of  the  region. 


FOLDINGS  OF  THE  CRUST 

The  oscillations  which  we  have  just  described  leave  the  strata 
not  far  from  their  original  horizontal  attitude.  Figure  167  repre- 
sents a  region  in  which  movements  of  a  very  different  nature 


FIG.  167.   Section  in  a  Region  of  Folded  Rocks 


202 


THE  ELEMENTS  OF  GEOLOGY 


have  taken  place.  Here,  on  either  side  of  the  valley  v,  we 
find  outcrops  of  layers  tilted  at  high  angles.  Sections  along 
the  ridge  r  show  that  it  is  composed  of  layers  which  slant 
inward  from  either  side.  In  places  the 
outcropping  strata  stand  nearly  on  edge, 
and  on  the  right  of  the  valley  they  are 
quite  overturned;  a  shale  sh  has  come 
FIG.  168.  Dip  and  Strike  to  overlie  a  limestone  Im,  although  the 
shale  is  the  older  rock,  whose  original  position  was  beneath  the 
limestone. 

It  is  not  reasonable  to  suppose  that  these  rocks  were  deposited 
in  the  attitude  in  which  we  find  them  now ;  we  must  believe 
that,  like  other  stratified  rocks,  they  were  outspread  in  nearly 
level  sheets  upon  the  ocean  floor.  Since  that  time  they  must 
have  been  deformed.  Layers  of  solid  rock  several  miles  in 
thickness  have  been  crumpled  and  folded  like  soft  wax  in  the 
hand,  and  a  vast  denudation  has  worn  away  the  upper  portions 
of  the  folds,  in  part  represented  in  our  section  by  dotted  lines. 

Dip  and  strike.  In 
districts  where  the 
strata  have  been  dis- 
turbed it  is  desirable  to 
record  their  attitude. 
This  is  most  easily  done 
by  taking  the  angle  at 
which  the  strata  are  in- 
clined and  the  compass 
direction  in  which  they 

slant.     It   is   also    con- 

FIG.  169.   An  Anticline,  Maryland 
vement  to  record  the 

direction  in  which  the  outcrop  of  the  strata  trends  across  the 
country. 

The  inclination  of  a  bed  of  rocks  to  the  horizon  is  its  dip 
(Fig.  168).  The  amount  of  the  dip  is  the  angle  made  with  a 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


203 


horizontal  plane.  The  dip  of  a  horizontal  layer  is  zero,  and  that 
of  a  vertical  layer  is  90°.  The  direction  of  the  dip  is  taken 
with  the  compass.  Thus  a  geologist's  notebook  in  describing 
the  attitude  of  outcropping  strata  contains  many  such  entries 
as  these :  dip  32°  north,  or  dip  8°  south  20°  west, —  meaning  in 
the  latter  case  that  the  amount  of  the  dip  is  8°  and  the  direc- 
tion of  the  dip  bears  20°  west  of  south. 


FIG.  170.  Folded  Strata,  Coast  of  England 
A  syncline  in  the  center,  with  an  anticline  on  either  side 

The  line  of  intersection  of  a  layer  with  the  horizontal  plane 
is  the  strike.  The  strike  always  runs  at  right  angles  to  the  dip. 

Dip  and  strike  may  be  illustrated  by  a  book  set  aslant  on  a  shelf. 
The  dip  is  the  acute  angle  made  with  the  shelf  by  the  side  of  the 
book,  while  the  strike  is  represented  by  a  line  running  along  the  book's 
upper  edge.  If  the  dip  is  north  or  south,  the  strike  runs  east  and  west. 

Folded  structures.  An  upfold,  in  which  the  strata  dip  away 
from  a  line  drawn  along  the  crest  and  called  the  axis  of  the 
fold,  is  known  as  an  anticline  (Fig.  169).  A  downfold,  where 
the  strata  dip  from  either  side  toward  the  axis  of  the  trough,  is 


204 


THE   ELEMENTS   OF   GEOLOGY 


called  a  syncline  (Fig.  170).  There  is  sometimes  seen  a  down- 
ward bend  in  horizontal  or  gently  inclined 'strata,  by  which  they 

descend  to  a  lower  level. 
Such  a  single  flexure  is  a 
monocline  (Fig.  171). 

Degrees  of  folding.  Folds 
vary  in  degree  from  broad, 
low  swells,  which  can  hard- 
ly be  detected,  to  the  most 
FIG.  171.  A  Monocline  highly  contorted  and  com- 

plicated structures.  In  symmetric  folds  (Figs.  169  and  180)  the 
dips  of  the  rocks  on  each  side  the  axis  of  the  fold  are  equal. 
In  unsymmetrical  folds  one  limb  is  steeper  than  the  other,  as 
in  the  anticline  in  Figure  167.  In  overturned  folds  (Figs.  167 
and  172)  one  limb  is  inclined  beyond  the  perpendicular.  Fan 
folds  have  been  so  pinched  that  the  original  anticlines  are  left 
broader  at  the  top  than  at  the  bottom  (Fig.  173). 


FIG.  172.   Overturned  Fold,  Vermont 

In  folds  where  the  compression  has  been  great  the  layers  are  often 
found  thickened  at  the  crest  and  thinned  along  the  limbs  (Fig.  174). 
Where  strong  rocks  such  as  heavy  limestones  are  folded  together  with 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


205 


weak  rocks  such  as  shales,  the  strong  rocks  are  often  bent  into  great 
simple  folds,  while  the  weak  rocks  are  minutely  crumpled. 

Systems  of  folds.  As  a  rule,  folds  occur  in  systems.  Over 
the  Appalachian  mountain  belt,  for  example,  extending  from 
northeastern  Pennsylvania  to  northern  Alabama  and  Georgia, 
the  earth's  crust  has 
been  thrown  into  a 
series  of  parallel  folds 
whose  axes  run  from 
northeast  to  south- 
west (Fig.  175).  In 
Pennsylvania  one  FlG' 17a  Fan  Folds' the  Alps 

may  count  a  score  or  more  of  these  earth  waves, —  some  but  from 
ten  to  twenty  miles  in  length,  and  some  extending  as  much  as 
two  hundred  miles  before  they  die  away.  On  the  eastern  part 
of  this  belt  the  folds  are  steeper  and  more  numerous  than  on 
the  western  side. 

Cause  and  conditions  of  folding.  The  sections  which  we  have 
studied  suggest  that  rocks  are  folded  by  lateral  pressure.  While 
a  single,  simple  fold  might  be  produced  by  a  heave,  a  series  of 
folds,  including  overturns,  fan  folds,  and  folds  thickened  on 
then-  crests  at  the  expense  of  their  limbs,  could  only  be  made 

hi  one  way,  —  by  pressure  from  the 
side.  Experiment  has  reproduced  all 
forms  of  folds  by  subjecting  to  lat- 
eral thrust  layers  of  plastic  material 
such  as  wax. 

Vast  as  the  force  must  have  been 
which  could  fold  the  solid  rocks  of 
the  crust  as  one  may  crumple  the 


FIG.  174.  Folds  with  Layers 
thickened  at  the  Crest 
and  thinned  along  the 
Limbs 


leaves  of  a  magazine  in  the  fingers,  it  is  only  under  certain  con- 
ditions that  it  could  have  produced  the  results  which  we  see. 
Rocks  are  brittle,  and  it  is  only  when  under  a  heavy  load,  and 
by  great  pressure  slowly  applied,  that  they  can  thus  be  folded 


206 


THE  ELEMENTS  OF   GEOLOGY 


FIG.  175.   Relief  Map  of  the  Northern  Appalachian  Region 
From  Brigham's  Geographic  Influences  in  American  History 

and  bent  instead  of  being  crushed  to  pieces.  Under  these  con- 
ditions, experiments  prove  that  not  only  metals  such  as  steel, 
but  also  brittle  rocks  such  as  marble,  can  be  deformed  and 
molded  and  made  to  flow  like  plastic  clay. 


MOVEMENTS  OF  THE  EARTH'S  CRUST      207 

Zone  of  flow,  zone  of  flow  and  fracture,  and  zone  of  fracture. 

We  may  believe  that  at  depths  which  must  be  reckoned  in  tens 
of  thousands  of  feet  the  load  of  overlying  rocks  is  so  great  that 
rocks  of  all  kinds  yield  by  folding  to  lateral  pressure,  and  flow 
instead  of  breaking.  Indeed,  at  such  profound  depths  and  under 
such  inconceivable  weight  no  cavity  can  form,  and  any  fractures 
would  be  healed  at  once  by  the  welding  of  grain  to  grain.  At 
less  depths  there  exists  a  zone  where  soft  rocks  fold  and  flow 
under  stress,  and  hard  rocks  are  fractured ;  while  at  and  near 
the  surface  hard  and  soft  rocks  alike  yield  by  fracture  to  strong 
pressure. 

STRUCTURES  DEVELOPED  IN  COMPRESSED  KOCKS 

Deformed  rocks  show  the  effects  of  the  stresses  to  which 
they  have  yielded,  not  only  in  the  immense  folds  into  which 
they  have  been  thrown  but  hi  their  smallest  parts  as  well. 
A  hand  specimen  of  slate,  or  even  a  particle  under  the  micro- 
scope, may  show  plications  similar  in  form  and  origin  to  the 
foldings  which  have  produced  ranges  of  mountains.  A  tiny 
flake  of  mica  in  the  rocks  of  the  Alps  may  be  puckered  by  the 
same  resistless  forces  which  have  folded  miles  of  solid  rock  to 
form  that  lofty  range. 

Slaty  cleavage.  Eocks  which  have  yielded  to  pressure  often 
split  easily  in  a  certain  direction  across  the  bedding  planes. 
This  cleavage  is  known  as  slaty  cleavage,  since  it  is  most  per- 
fectly developed  in  fine-grained,  homogeneous  rocks,  such  as 
slates,  which  cleave  to  the  thin,  smooth-surfaced  plates  with 
which  we  are  familiar  in  the  slates  used  in  roofing  and  for 
ciphering  and  blackboards.  In  coarse-grained  rocks,  pressure 
develops  more  distant  partings  which  separate  the  rocks  into 
blocks. 

Slaty  cleavage  cannot  be  due  to  lamination,  since  it  commonly 
crosses  bedding  planes  at  an  angle,  while  these  planes  have 
been  often  well-nigh  or  quite  obliterated.  Examining  slate  with 


208  THE  ELEMENTS  OF  GEOLOGY 

a  microscope,  we  find  that  its  cleavage  is  due  to  the  grain  of 
the  rock.  Its  particles  are  flattened  and  lie  with  their  broad 
faces  in  parallel  planes,  along  which  the  rock  naturally  splits 
more  easily  than  in  any  other  direction.  The  irregular  grains 
of  the  mud  which  has  been  altered  to  slate  have  been  squeezed 

flat  by  a  pressure  ex- 
erted at  right  angles 
to.  the  plane  of  cleav- 
age.  Cleavage  is 
found  only  in  folded 
rocks,  and,  as  we 

S6e 


Fio.176.   Slaty  Cleavage 

176,  the  strike  of  the 

cleavage  runs  parallel  to  the  strike  of  the  strata  and  the  axis 
of  the  folds.  The  dip  of  the  cleavage  is  generally  steep,  hence 
the  pressure  was  nearly  horizontal.  The  pressure  which  has 
acted  at  right  angles  to  the  cleavage,  and  to  which  it  is  due,  is 
the  same  lateral  pressure  which  has  thrown  the  strata  into  folds. 

We  find  additional  proof  that  slates  have  undergone  compression  at 
right  angles  to  their  cleavage  in  the  fact  that  any  inclusions  in  them, 
such  as  nodules  and  fossils,  have  been  squeezed  out  of  shape  and  have 
their  long  diameters  lying  in  the  planes  of  cleavage. 

That  pressure  is  competent  to  cause  cleavage  is  shown  by  experi- 
ment. Homogeneous  material  of  fine  grain,  such  as  beeswax,  when 
subjected  to  heavy  pressure  cleaves  at  right  angles  to  the  direction  of 
the  compressing  force. 

Rate  of  folding.  All  the  facts  known  with  regard  to  rock 
deformation  agree  that  it  is  a  secular  process,  taking  place  so 
slowly  that,  like  the  deepening  of  valleys  by  erosion,  it  escapes 
the  notice  of  the  inhabitants  of  the  region.  It  is  only  under 
stresses  slowly  applied  that  rocks  bend  without  breaking.  The 
folds  of  some  of  the  highest  mountains  have  risen  so  gradually 
that  strong,  well-intrenched  rivers  which  had  the  right  of  way 
across  the  region  were  able  to  hold  to  their  courses,  and  as 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


209 


a  circular  saw  cuts  its  way  through  the  log  which  is  steadily 
driven  against  it,  so  these  rivers  sawed  their  gorges  through 
the  fold  as  fast  as  it  rose  beneath  them.  Streams  which  thus 
maintain  the  course  which  they  had  antecedent  to  a  deforma- 
tion of  the  region  are  known  as  antecedent  streams.  Examples 
of  such  are  the  Sutlej  and  other  rivers  of  India,  whose  valleys 
trench  the  outer  ranges  of  the  Himalayas  and  whose  earlier 
river  deposits  have  been  upturned  by  the  rising  ridges.  On 
the  other  hand,  mountain  crests  are  usually  divides,  parting  the 
head  waters  of  different  drainage  systems.  In  these  cases  the 
original  streams  of  the  region  have  been  broken  or  destroyed  by 
the  uplift  of  the  mountain  mass  across  their  paths. 

On  the  whole,  which  have  worked  more  rapidly,  processes  of  defor- 
mation or  of  denudation  ? 


LAND  FORMS  DUE  TO  FOLDING 

As  folding  goes  on  so  slowly,  it  is  never  left  to  form  surface 
features  unmodified  by  the  action  of  other  agencies.  An  anti- 
clinal fold  is  attacked  by 
erosion  as  soon  as  it  begins 
to  rise  above  the  original 
level,  and  the  higher  it  is 
uplifted,  and  the  stronger 
are  its  slopes,  the  faster  is 
it  worn  away.  Even  while 
rising,  a  young  upfold  is 
often  thus  unroofed,  and 
instead  of  appearing  as  a 
long,  smooth,  boat-shaped  FlG-  m-  An  Unroofed  Anticline 

ridge,  it  commonly  has  had  opened  along  the  rocks  of  the  axis, 
when  these  are  weak,  a  valley  which  is  overlooked  by  the  in- 
facing  escarpments  of  the  hard  layers  of  the  sides  of  the  fold 
(Fig.  177).  Under  long-continued  erosion,  anticlines  may  be 


210 


THE  ELEMENTS  OF   GEOLOGY 


degraded  to  valleys,  while  the  synclines  of  the  same  system 
may  be  left  in  relief  as  ridges  (Fig.  167). 

Folded  mountains.  The  vastness  of  the  forces  which  wrin- 
kle the  crust  is  best  realized  in  the  presence  of  some  lofty 
mountain  range.  All  mountains,  indeed,  are  not  the  result  of 
folding.  Some,  as  we  shall  see,  are  due  to  upwarps  or  to  frac- 
tures of  the  crust ;  some  are  piles  of  volcanic  material ;  some 


FIG.  178.    Mountain  Peaks  carved  in  Folded  Strata,  Rocky 
Mountains,  Montana 

are  swellings  caused  by  the  intrusion  of  molten  matter  beneath 
the  surface ;  some  are  the  relicts  left  after  the  long  denudation 
of  high  plateaus. 

But  most  of  the  mountain  ranges  of  the  earth,  and  some  of 
the  greatest,  such  as  the  Alps  and  the  Himalayas,  were  origi- 
nally mountains  of  folding.  The  earth's  crust  has  wrinkled  into 
a  fold ;  or  into  a  series  of  folds,  forming  a  series  of  parallel 
ridges  and  intervening  valleys  ;  or  a  number  of  folds  have  been 
mashed  together  into  a  vast  upswelling  of  the  crust,  in  which 
the  layers  have  been  so  crumpled  and  twisted,  overturned  and 


MOVEMENTS  OF  THE  EARTH'S  CRUST      211 

crushed,  that  it  is  exceedingly  difficult  to  make  out  the  origi- 
nal structure. 

The  close  and  intricate  folds  seen  in  great  mountain  ranges 
were  formed,  as  we  have  seen,  deep  below  the  surface,  within  the 
zone  of  folding.  Hence  they  may  never  have  found  expression 
in  any  individual  surface  features.  As  the  result  of  these  defor- 
mations deep  under  ground  the  surface  was  broadly  lifted  to 
mountain  height,  and  the  crumpled  and  twisted  mountain 


FIG.  179.  Section  of  a  Portion  of  the  Alps 

structures  are  now  to  be  seen  only  because  erosion  has  swept 
away  the  heavy  cover  of  surface  rocks  under  whose  load  they 
were  developed. 

When  the  structure  of  mountains  has  been  deciphered  it  is  possible 
to  estimate  roughly  the  amount  of  horizontal  compression  which  the 
region  has  suffered.  If  the  strata  of  the  folds  of  the  Alps  were  smoothed 
out,  they  would  occupy  a  belt  from  four  hundred  to  seven  hundred  and 
fifty  miles  wide,  while  the  width  to  which  they  have  been  compressed 
is  but  one  hundred  miles.  A  section  across  the  Appalachian  folds  in 
Pennsylvania  shows  a  compression  to  about  two  thirds  the  original 
width ;  the  belt  has  been  shortened  thirty-five  miles  in  every  hundred. 

Considering  the  thickness  of  their  strata,  the  compression  which  moun- 
tains have  undergone  accounts  fully  for  their  height,  with  enough  to 
spare  for  all  that  has  been  lost  by  denudation. 

The  Appalachian  folds  involve  strata  thirty  thousand  feet  in  thick- 
ness. Assuming  that  the  folded  strata  rested  on  an  unyielding  founda- 
tion, and  that  what  was  lost  in  width  was  gained  in  height,  what  elevation 
would  the  range  have  reached  had  not  denudation  worn  it  as  it  rose  ? 


212  THE  ELEMENTS  OF  GEOLOGY 

The  life  history  of  mountains.  While  the  disturbance  and 
uplift  of  mountain  masses  are  due  to  deformation,  their  sculp- 
ture into  ridges  and  peaks,  valleys  and  deep  ravines,  and  all 
the  forms  which  meet  the  eye  in  mountain  scenery,  excepting 
in  the  very  youngest  ranges,  is  due  solely  to  erosion.  We  may 
therefore  classify  mountains  according  to  the  degree  to  which 
they  have  been  dissected.  The  Juras  are  an  example  of  the 
stage  of  early  youth,  in  which  the  anticlines  still  persist  as  ridges 
and  the  synclines  coincide  with  the  valleys ;  this  they  owe  as 
much  to  the  slight  height  of  their  uplift  as  to  the  recency  of  its 
date  (Fig.  180). 

The  Alps  were  upheaved  at  various  times  (p.  399),  the  last 
uplift  being  later  than  the  uplift  of  the  Juras,  but  to  so  much 
greater  height  that  erosion  has  already  advanced  them  well  on 


FIG.  180.   Section  of  a  Portion  of  the  Jura  Mountains 

towards  maturity.  The  mountain  mass  has  been  cut  to  the  core, 
revealing  strange  contortions  of  strata  which  could  never  have 
found  expression  at  the  surface.  Sharp  peaks,  knife-edged  crests, 
deep  valleys  with  ungraded  slopes  subject  to  frequent  landslides, 
are  all  features  of  Alpine  scenery  typical  of  a  mountain  range 
at  this  stage  in  its  life  history.  They  represent  the  survival  of 
the  hardest  rocks  and  the  strongest  structures,  and  the  destruc- 
tion of  the  weaker  in  their  long  struggle  for  existence  against 
the  agents  of  erosion.  Although  miles  of  rock  have  been  re- 
moved from  such  ranges  as  the  Alps,  we  need  not  suppose  that 
they  ever  stood  much,  if  any,  higher  than  at  present.  All  this 
vast  denudation  may  easily  have  been  accomplished  while  their 
slow  upheaval  was  going  on ;  in  several  mountain  ranges  we 
have  evidence  that  elevation  has  not  yet  ceased. 

Under  long  denudation  mountains  are  subdued  to  the  forms 
characteristic  of  old  age.    The  lofty  peaks  and  jagged  crests  of 


213 


214 


THE  ELEMENTS   OF   GEOLOGY 


their  earlier  life  are  smoothed  down  to  low  domes  and  rounded 
crests.  The  southern  Appalachians  and  portions  of  the  Hartz 
Mountains  in  Germany  (Fig.  182)  are  examples  of  mountains 
which  have  reached  this  stage. 

There  are  numerous  regions  of  upland  and  plains  in  which 
the  rocks  are  found  to  have  the  same  structure  that  we  have 
seen  in  folded  mountains ;  they  are  tilted,  crumpled,  and  over- 
turned, and  have  clearly  suffered  intense  compression.  We  may 


FIG.  182.   Subdued  Mountains,  the  Hartz  Mountains,  Germany 

infer  that  their  folds  were  once  lifted  to  the  height  of  mountains 
and  have  since  been  wasted  to  low-lying  lands.  Such  a  section 
as  that  of  Figure  67  illustrates  how  ancient  mountains  may  be 
leveled  to  their  roots,  and  represents  the  final  stage  to  which 
even  the  Alps  and  the  Himalayas  must  sometime  arrive. 
Mountains,  perhaps  of  Alpine  height,  once  stood  about  Lake 
Superior ;  a  lofty  range  once  extended  from  New  England  and 
New  Jersey  southwestward  to  Georgia  along  the  Piedmont  belt. 
In  our  study  of  historic  geology  we  shall  see  more  clearly  how 


MOVEMENTS  OF  THE  EARTH'S  CRUST  215 

short  is  the  life  of  mountains  as  the  earth  counts  time,  and  how 
great  ranges  have  been  lifted,  worn  away,  and  again  upheaved 
into  a  new  cycle  of  erosion. 

The  sedimentary 'history  of  folded  mountains.  We  may  men- 
tion here  some  of  the  conditions  which  have  commonly  been 
antecedent  to  great  foldings  of  the  crust. 

1.  Mountain  ranges  are  made  of  belts  of  enormously  and 
exceptionally  thick  sediments.    The  strata  of  the  Appalachians 
are  thirty  thousand  feet  thick,  while  the  same  formations  thin 
out  to  five  thousand  feet  in  the  Mississippi  valley.    The  folds  of 
the  Wasatch  Mountains  involve  strata  thirty  thousand  feet  thick, 
which  thin  to  two  thousand  feet  in  the  region  of  the  Plains. 

2.  The  sedimentary  strata  of  which  mountains  are  made  are 
for  the  most  part  the  shallow-water  deposits  of  continental 
deltas.    Mountain  ranges  have  been  upfolded  along  the  margins 
of  continents. 

3.  Shallow-water  deposits  of  the  immense  thickness  found 
in  mountain  ranges  can  be  laid  only  in  a  gradually  sinking  area. 
A  profound  subsidence,  often  to  be  reckoned  in  tens  of  thou- 
sands of  feet,  precedes  the  upfolding  of  a  mountain  range. 

Thus  the  history  of  mountains  of  folding  is  as  follows :  For 
long  ages  the  sea  bottom  off  the  coast  of  a  continent  slowly 
subsides,  and  the  great  trough,  as  fast  as  it  forms,  is  filled  with 
sediments,  which  at  last  come  to  be  many  thousands  of  feet 
thick.  The  downward  movement  finally  ceases.  A  slow  but 
resistless  pressure  sets  in,  and  gradually,  and  with  a  long  series 
of  many  intermittent  movements,  the  vast  mass  of  accumulated 
sediments  is  crumpled  and  uplifted  into  a  mountain  range. 

FRACTURES  AND  DISLOCATIONS  OF  THE  CRUST 

Considering  the  immense  stresses  to  which  the  rocks  of  the 
crust  are  subjected,  it  is  not  surprising  to  find  that  they  often 
yield  by  fracture,  like  brittle  bodies,  instead  of  by  folding  and 


216  THE  ELEMENTS  OF  GEOLOGY 

flowing,  like  plastic  solids.  Whether  rocks  bend  or  break  de- 
pends on  the  character  and  condition  of  the  rocks,  the  load  of 
overlying  rocks  which  they  bear,  and  the  amount  of  the  force 
and  the  slowness  with  which  it  is  applied.  * 

Joints.  At  the  surface,  where  their  load  is  least,  we  find  rocks 
universally  broken  into  blocks  of  greater  or  less  size  by  partings 
known  as  joints.  Under  this  name  are  included  many  division 
planes  caused  by  cooling  and  drying;  but  it  is  now  generally 
believed  that  the  larger  and  more  regular  joints,  especially  those 


FIG.  183.  Joints  utilized  by  a  River  in  widening  its  Valley,  Iowa 

which  run  parallel  to  the  dip  and  strike  of  the  strata,  are  frac- 
tures due  to  up-and-down  movements  and  foldings  and  twistings 
of  the  rocks. 

Joints  are  used  to  great  advantage  in  quarrying,  and  we  have 
seen  how  they  are  utilized  by  the  weather  in  breaking  up  rock 
masses,  by  rivers  in  widening  their  valleys,  by  the  sea  in  driving 
back  its  cliffs,  by  glaciers  in  plucking  their  beds,  and  how  they 
are  enlarged  in  soluble  rocks  to  form  natural  passageways  for 
underground  waters.  The  ends  of  the  parted  strata  match  along 


MOVEMENTS  OF  THE  EARTH'S  CRUST      217 

both  sides  of  joint  planes ;  in  joints  there  has  been  little  or  no 
displacement  of  the  broken  rocks. 

Faults.  In  Figure  184  the  rocks  have  been  both  broken  and 
dislocated  along  the  plane  ff.  One  side  must  have  been  moved 
up  or  down  past  the  other.  Such  a  dislocation  is  called  a  fault. 
The  amount  of  the  displacement,  as  measured  by  the  vertical 
distance  between  the  ends  of  a  parted  layer,  is  the  throw  (cd). 
The  angle  (ffv)  which  the  fault  plane  ^.  v 

makes  with  the  vertical  is  the  hade. 
In  Figure  184  the  right  side  has  gone 
down  relatively  to  the  left;  the  right 
is  the  side  of  the  downthrow,  while 
the  left  is  the  side  of  the  upthrow. 
Where  the  fault  plane  is  not  vertical  the 
surfaces  on  the  two  sides  may  be  dis- 
tinguished as  the  hanging  wall  (that  on  the  right  of  Figure  184) 
and  the  foot  wall  (that  on  the  left  of  the  same  figure).  Faults 
differ  in  throw  from  a  fraction  of  an  inch  to  many  thousands 
of  feet. 

Slicken^ides.  If  we  examine  the  walls  of  a  fault,  we  may  find  further 
evidence  of  movement  in  the  fact  that  the  surfaces  are  polished  and 
grooved  by  the  enormous  friction  which  they  have  suffered  as  they 
have  ground  one  upon  the  other.  These  appearances,  called  slicken- 
sides,  have  sometimes  been  mistaken  for  the  results  of  glacial  action. 

Normal  faults.  Faults  are  of  two  kinds,  —  normal  faults 
and  thrust  faults.  Normal  faults,  of  which  Figure  184  is  an 
example,  hade  to  the  downthrow;  the  hanging  wall  has  gone 
down.  The  total  length  of  the  strata  has  been  increased  by  the 
displacement.  It  seems  that  the  strata  have  been  stretched  and 
broken,  and  that  the  blocks  have  readjusted  themselves  under 
the  action  of  gravity  as  they  settled. 

Thrust  faults.  Thrust  faults  hade  to  the  upthrow;  the 
hanging  wall  has  gone  up.  Clearly  such  faults,  where  the  strata 
occupy  less  space  than  before,  are  due  to  lateral  thrust.  Folds 


218 


THE  ELEMENTS  OF  GEOLOGY 


FIG.  185.   A  Thrust  Fault 


and  thrust  faults  are  closely  associated. .  Under  lateral  pressure 
strata  may  fold  to  a  certain  point  and  then  tear  apart  and  fault 
along  the  surface  of  least  resistance.  Under  immense  pressure 
strata  also  break  by  shear  without  folding.  Thus,  in  Figure  185, 
the  rigid  earth  block  under  lateral  thrust  has  found  it  easier  to 
break  along  the  fault  plane  than  to  fold.  Where  such  faults  are 

nearly  horizontal  they  are 
distinguished  as  thrust 
planes. 

In  all  thrust  faults  one 
mass  has  been  pushed  over 
another,  so  as  to  bring  the 

underlying  and  older  strata  upon  younger  beds ;  and  when  the 
fault  planes  are  nearly  horizontal,  and  especially  when  the  rocks 
have  been  broken  into  many  slices  which  have  slidden  far  one 
upon  another,  the  true  succession  of  strata  is  extremely  hard  to 
decipher. 

In  the  Selkirk  Mountains  of  Canada  the  basement  rocks  of  the  region 
have  been  driven  east  for  seven  miles  on  a  thrust  plane,  over  rocks 
which  originally  lay  thousands  of  feet  above  them.  , 

Along  the  western  Appalachians,  from  Virginia  to  Georgia,  the 
mountain  folds  are  broken  by  more  than  fifteen  parallel  thrust  planes, 
running  from  northeast  to  southwest,  along  which  the  older  strata  have 
been  pushed  westward  over  the  younger.  The  longest  continuous  fault 
has  been  traced  three  hundred  and  seventy-five  miles,  and  the  greatest 
horizontal  displacement  has  been  estimated  at  not  less  than  eleven  miles. 

Crush  breccia.  Eocks  often  do  not  fault  with  a  clean  and 
simple  fracture,  but  along  a  zone,  sometimes  several  yards  in 
width,  in  which  they  are  broken  to  fragments.  It  may  occur 
also  that  strata  which  as  a  whole  yield  to  lateral  thrust  by 
folding  include  beds  of  brittle  rocks,  such  as  thin-layered  lime- 
stones, which  are  crushed  to  pieces  by  the  strain.  In  either 
case  the  fragments  when  recemented  by  percolating  waters  form 
a  rock  known  as  a  crush  breccia  (pronounced  bretcha)  (Fig.  186). 


MOVEMENTS  OF  THE  EARTH'S  CRUST      219 

Breccia  is  a  term  applied  to  any  rock  formed  of  cemented 
angular  fragments.  This  rock  may  be  made  by  the  consolida- 
tion of  volcanic  cinders,  of  angular  waste  at  the  foot  of  cliffs,  or 
of  fragments  of  coral  torn  by  the  waves  from  coral  reefs,  as  well 
as  of  strata  crushed  by  crustal  movements. 

SURFACE  FEATURES  DUE  TO  DISLOCATIONS 

Fault  scarps.  A  fault  of  recent  date  may  be  marked  at  sur- 
face by  a  scarp,  because  the  face  of  ,the  upthrown  block  has  not 
yet  been  worn  to  the 
level  of  the  down- 
throw side. 

After  the  upthrown 
block  has  been  worn 
down  to  this  level, 
differential  erosion 
produces  fault  scarps 
wherever  weak  rocks 
and  resistant  rocks 
are  brought  in  con- 
tact along  the  fault 
plane ;  and  the  harder 
rocks,  whether  on  the 
upthrow  or  the  down- 
throw side,  emerge 

,.  e      ,.„,  FIG.  186.   Breccia 

in    a    line   ot    dins. 

Where  a  fault  is  so  old  that  no  abrupt  scarps  appear,  its  general 
course  is  sometimes  marked  by  the  line  of  division  between 
highland  and  lowland  or  hill  and  plain.  Great  faults  have  some- 
times brought  ancient  crystalline  rocks  in  contact  with  weaker 
and  younger  sedimentary  rocks,  and  long  after  erosion  has  de- 
stroyed all  fault  scarps  the  harder  crystallines  rise  in  an  upland 
of  rugged  or  mountainous  country  which  meets  the  lowland 
along  the  line  of  faulting. 


220 


THE  ELEMENTS  OF  GEOLOGY 


The  vast  majority  of  faults  give  rise  to  no  surface  features. 
The  faulted  region  may  be  old  enough  to  have  been  baseleveled, 
or  the  rocks  on  both  sides  of  the  line  of  dislocation  may  be 
alike  in  their  resistance  to  erosion  and  therefore  have  been 

worn  down  to  a  common  slope. 
The  fault  may  be  entirely  con- 
cealed by  the  mantle  of  waste, 
and  in  such  cases  it  can  be  in- 
ferred from  abrupt  changes  in 
the  character  or  the  strike  and 
dip  of  the  strata  where  they 

may  outcrop  near  it  (Fig.  187). 
FIG.  187.  A  Concealed  Fault 

This  fault  may  be  inferred  from  the  The  plateau  trenched  by  the 
changes  in  strata  in  passing  along  the  Qrand  Canyon  of  the  Colorado 
strike,  as  from  6  to  a  and  from  c  to  6  . 

River  exhibits  a  series  of  mag- 
nificent fault  scarps  whose  general  course  is  from  north  to  south,  mark- 
ing the  edges  of  the  great  crust  blocks  into  which  the  country  has  been 
broken.  The  highest  part  of  the  plateau  is  a  crust  block  ninety  miles 
long  arid  thirty-five  miles  in  maximum  width,  which  has  been  hoisted 
to  nine  thousand  three  hundred  feet  above  sea  level.  On  the  east  it 
descends  four  thousand  feet  by  a  monoclinal  fold,  which  passes  into  a 
fault  towards  the  north.  On  the  west  it  breaks  down  by  a  succession  of 


FIG.  188.  East-West  Section  across  the  Broken  Plateau  north  of  the 
Grand  Canyon  of  the  Colorado  River,  Arizona 

terraces  faced  by  fault  scarps.  The  throw  of  these  faults  varies  from 
seven  hundred  feet  to  more  than  a  mile.  The  escarpments,  however, 
are  due  in  a  large  degree  to  the  erosion  of  weaker  rock  on  the  down- 
throw side. 

The  Highlands  of  Scotland  (Fig.  189)  meet  the  Lowlands  on  the  south 
with  a  bold  front  of  rugged  hills  along  a  line  of  dislocation  which  runs 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


221 


across  the  country  from  sea  to  sea.    On  the  one  side  are  hills  of  ancient 

crystalline  rocks  whose  crumpled  structures  prove  that  they  are  but  the 

roots  of  once  lofty  mountains ;  on  the  other  lies  a  lowland  of  sandstone 

and  other  stratified  rocks  formed  from  the  waste  of  those  long-vanished 

mountain  ranges.    Remnants  of  sandstone 

occur  in  places  on  the  north  of  the  great 

fault,  and  are  here  seen  to  rest  on  the  worn 

and  fairly  even  surface  of  the  crystallines. 

We  may  infer  that  these  ancient  mountains 

were  reduced  along  their   margins  to  low    FIG.  189.  The  Fault  separat- 

plains,  which  were  slowly  lowered  beneath 

the  sea  to  receive  a  cover  of  sedimentary 


ing  the  Highlands  and  the 
Lowlands,  Scotland 


rocks.  Still  later  came  an  uplift  and  dislocation.  On  the  one  side 
erosion  has  since  stripped  off  the  sandstones  for  the  most  part,  but  the 
hard  crystalline  rocks  yet  stand  in  bold  relief.  On  the  other  side  the 
weak  sedimentary  rocks  have  been  worn  down  to  lowlands. 

Rift  valleys.  In  a  broken  region  undergoing  uplift  or  the 
unequal  settling  which  may  follow,  a  slice  inclosed  between  two 
fissures  may  sink  below  the  level  of  the  crust  blocks  on  either 
side,  thus  forming  a  linear  depression  known  as  a  rift  valley,  or 
valley  of  fracture. 

One  of  the  most  striking  examples  of  this  rare  type  of  valley  is  the 
long  trough  which  runs  straight  from  the  Lebanon  Mountains  of  Syria 
on  the  north  to  the  Red  Sea  on  the  south,  and  whose  central  portion  is 

occupied  by  the  Jor- 
dan valley  and  the 
Dead  Sea.  The  pla- 
teau which  it  gashes 
has  been  lifted  more 
than  three  thousand 
feet  above  sea  level, 


a,  ancient  schists;  6,  Carboniferous  strata;  c,  d,  and 
e,  Cretaceous  strata 


FIG.  190.   Section  from  the  Mountains  of  Palestine 

to  the  Mountains  of  Moab  across  the  Dead  Sea 

sea 

and  the  bottom  of  the 
trough  reaches  a  depth 
of  two  thousand  six  hundred  feet  below  that  level  in  parts  of  the  Dead 
Sea.  South  of  the  Dead  Sea  the  floor  of  the  trough  rises  somewhat 
above  sea  level,  and  in  the  Gulf  of  Akabah  again  sinks  below  it.  This 
uneven  floor  could  be  accounted  for  either  by  the  profound  warping  of 


222         THE  ELEMENTS  OF  GEOLOGY 

a  valley  of  erosion  or  by  the  unequal  depression  of  the  floor  of  a  rift 
valley.  But  that  the  trough  is  a  true  valley  of  fracture  is  proved  by 
the  fact  that  on  either  side  it  is  bounded  by  fault  scarps  and  monoclinal 
folds.  The  keystone  of  the  arch  has  subsided.  Many  geologists  believe 
that  the  Jordan -Akabah  trough,  the  long  narrow  basin  of  the  Red  Sea, 
and  the  chain  of  down-faulted  valleys  which  in  Africa  extends  from  the 
strait  of  Bab-el-Mandeb  as  far  south  as  Lake  Nyassa  —  valleys  which 
contain  more  than  thirty  lakes  —  belong  to  a  single  system  of  dislocation. 
Should  you  expect  the  lateral  valleys  of  a  rift  valley  at  the  time  of 
its  formation  to  enter  it  as  hanging  valleys  or  at  a  common  level? 

Block  mountains.  Dislocations  take  place  on  so  grand  a 
scale  that  by  the  upheaval  of  blocks  of  the  earth's  crust  or  the 
down-faulting  of  the  blocks  about  one  which  is  relatively  sta- 
tionary, mountains  known  as  block  mountains  are  produced. 
A  tilted  crust  block  may  present  a  steep  slope  on  the  side  up- 
heaved and  a  more  gentle  descent  on  the  side  depressed. 

The  Basin  ranges.  The  plateaus  of  the  United  States  bounded  by  the 
Rocky  Mountains  on  the  east,  and  on  the  west  by  the  ranges  which 
front  the  Pacific,  have  been  profoundly  fractured  arid  faulted.  The 

system  of  great  fissures 
by  which  they  are  broken 
extends  north  and  south, 
and  the  long,  narrow, 
tilted  crust  blocks  inter- 
EIG.  191.  Block  Mountains,  Southern  Oregon  cepted  between  the  fis- 
sures give  rise  to  the 

numerous  north-south  ranges  of  the  region.  Some  of  the  tilted  blocks, 
as  those  of  southern  Oregon,  are  as  yet  but  moderately  carved  by  ero- 
sion, and  shallow  lakes  lie  on  the  waste  that  has  been  washed  into  the 
depressions  between  them  (Fig.  191).  We  may  therefore  conclude  that 
their  displacement  is  somewhat  recent.  Others,  as  those  of  Nevada,  are 
so  old  that  they  have  been  deeply  dissected ;  their  original  form  has 
been  destroyed  by  erosion,  and  the  intermontane  depressions  are  occupied 
by  wide  plains  of  waste. 

Dislocations  and  river  valleys.  Before  geologists  had  proved 
that  rivers  can  by  their  own  unaided  efforts  cut  deep  canyons,  it 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


223 


was  common  to  consider  any  narrow  gorge  as  a  gaping  fissure 
of  the  crust.  This  crude  view  has  long  since  been  set  aside. 
A  map  of  the  plateaus  of  northern  Arizona  shows  how  inde- 
pendent of  the  immense  faults  of  the  region  is  the  course  of  the 
Colorado  River.  In  the  Alps  the  tunnels  on  the  Saint  Gott- 
hard  railway  pass  six  times  beneath  the  gorge  of  the  Reuss,  but 
at  no  point  do  the  rocks  show  the  slightest  trace  of  a  fault. 


FIG.  192.   Fault  crossing  Valley  in  Japan 

Rate  of  dislocation.  So  far  as  human  experience  goes,  the 
earth  movements  which  we  have  just  studied,  some  of  which 
have  produced  deep-sunk  valleys  and  lofty  mountain  ranges, 
and  faults  whose  throw  is  to  be  measured  in  thousands  of  feet, 
are  slow  and  gradual.  They  are  not  accomplished  by  a  single 
paroxysmal  effort,  but  by  slow  creep  and  a  series  of  slight  slips 
continued  for  vast  lengths  of  time. 

In  the  Aspen  mining  district  in  Colorado  faulting  is  now  going  on 
at  a  comparatively  rapid  rate.  Although  no  sudden  slips  take  place,  the 
creep  of  the  rock  along  certain  planes  of  faulting  gradually  bends  out 


224  THE  ELEMENTS  OF  GEOLOGY 

of  shape  the  square-set  timbers  in  horizontal  drifts  and  has  closed 
some  vertical  shafts  by  shifting  the  upper  portion  across  the  lower. 
Along  one  of  the  faults  of  this  region  it  is  estimated  that  there  has 
been  a  movement  of  at  least  four  hundred  feet  since  the  Glacial  epoch. 
More  conspicuous  are  the  instances  of  active  faulting  by  means  of 
sudden  slips.  In  1891  there  occurred  along  an  old  fault  plane  in  Japan 
a  slip  which  produced  an  earth  rent  traced  for  fifty  miles  (Fig.  192). 
The  country  on  one  side  was  depressed  in  places  twenty  feet  below 
that  on  the  other,  and  also  shifted  as  much  as  thirteen  feet  horizon- 
tally in  the  direction  of  the  fault  line. 

In  1872  a  slip  occurred  for  forty  miles  on  the  great  line  of  dislocation 
which  runs  along  the  eastern  base  of  the  Sierra  Nevada  Mountains. 
In  the  Owens  valley,  California,  the  throw  amounted  to  twenty-five  feet 
in  places,  with  a  horizontal  movement  along  the  fault  line  of  as  much  as 
eighteen  feet.  Both  this  slip  and  that  in  Japan  just  mentioned  caused 
severe  earthquakes. 

For  the  sake  of  clearness  we  have  described  oscillations,  fold- 
ings, and  fractures  of  the  crust  as  separate  processes,  each  giv- 
ing rise  to  its  own  peculiar  surface  features,  but  in  nature 
earth  movements  are  by  no  means  so  simple, —  they  are  often 
implicated  with  one  another :  folds  pass  into  faults ;  in  a 
deformed  region  certain  rocks  have  bent,  while  others  under  the 
same  strain,  but  under  different  conditions  of  plasticity  and 
load,  have  broken ;  folded  mountains  have  been  worn  to  their 
roots,  and  the  peneplains  to  which  they  have  been  denuded 
have  been  upwarped  to  mountain  height  and  afterwards  dis- 
sected,—  as  in  the  case  of  the  Alleghany  ridges,  the  southern 
Carpathians,  and  other  ranges,  —  or,  as  in  the  case  of  the  Sierra 
Nevada  Mountains,  have  been  broken  and  uplifted  as  mountains 
of  fracture. 

Draw  the  following  diagrams,  being  careful  to  show  the  direction  in 
which  the  faulted  blocks  have  moved,  by  the  position  of  the  two  parts 
of  some  well-defined  layer  of  limestone,  sandstone,  or  shale,  which 
occurs  on  each  side  of  the  fault  plane,  as  in  Figure  184. 

1.  A  normal  fault  with  a  hade  of  15°,  the  original  fault  scarp 
remaining. 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


225 


2.  A  normal  fault  with  a  hade  of  50°,  the  original  fault  scarp  worn 
away,  showing  cliffs  caused  by  harder  strata  on  the  downthrow  side. 

3.  A  thrust  fault  with  a  hade  of  30°,  showing  cliffs  due  to  harder 
strata  outcropping  on  the  downthrow. 

4.  A  thrust  fault  with  a  hade  of  80°,  with  surface  baseleveled. 

5.  In  a  region  of  normal  faults  a  coal  mine  is  being  worked  along 
the  seam  of  coal  AB  (Fig.  193).    At  B  it  is  found  broken 

by  a  fault /which  hades  toward  A.     To  find  the  seam 
again,  should  you  advise  tunneling  up  or  down  from  B1 

6.  In  a  vertical  shaft  of  a  coal  mine  the  same  bed  of 
coal  is  pierced  twice  at  different  levels  because  of  a  fault. 
Draw  a  diagram  to  show  whether  the  fault  is  normal  or 


L 


'n 

FIG.  193 
i  thrust. 


FIG.  194.   Ridges  to  be  explained  by  Faulting 

7.  Copy  the  diagram  in  Figure  194,  showing  how  the  two  ridges  may 
be  accounted  for  by  a  single  resistant  stratum  dislocated  by  a  fault.  Is 
the  fault  a  strike  fault,  i.e.  one  running  parallel  with  the  strike  of  the 
strata,  "or  a  dip  fault,  one  running  parallel  with  the  direction  of  the  dip? 


FIG.  195.  Earth  Block  of  Tilted  Strata,  with  Included  Seam  of  Coal  cc 

8.  Draw  a  diagram  of  the  block  in  Figure  195  as  it  would  appear  if 
dislocated  along  the  plane  efg  by  a  normal  fault  whose  throw  equals 
one  fourth  the  height  of  the  block.  Is  the  fault  a  strike  or  a  dip  fault? 


226 


THE  ELEMENTS  OF  GEOLOGY 


Draw  a  second  diagram  showing  the  same  block  after  denudation  has 
worn  it  down  below  the  center  of  the  upthrown  side.  Note  that  the  out- 
crop of  the  coal  seam  is  now  deceptively  repeated.  This  exercise  may 
be  done  in  blocks  of  wood  instead  of  drawings. 


FIG.  196,  A  and  B.   Repeated  Outcrops  of  the  Same  Strata 

9.  Draw  diagrams  showing  by  dotted  lines  the  conditions  both  of  A 
and  of  B,  Figure  196,  after  deformation  had  given  the  strata  their  pres- 
ent attitude. 


FIG.  197.    A  Block  Mountain 

10.  What  is  the  attitude  of  the  strata  of  this  earth  block,  Figure  197? 
What  has  taken  place  along  the  plane  ftq/"?  When  did  the  dislocation 
occur  compared  with  the  folding  of  the  strata  ?  with  the  erosion  of  the 
valleys  on  the  right-hand  side  of  the  mountain?  with  the  deposition  of 
the  sediments  efgl  Do  you  find  any  remnants  of  the  original  surface 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


227 


baf  produced  by  the  dislocation?  From  the  left-hand  side  of  the  moun- 
tain infer  what  was  the  relief  of  the  region  before  the  dislocation.  Give 
the  complete  history  recorded  in  the  diagram  from  the  deposition  of  the 
strata  to  the  present. 


FIG.  198.   A  Faulted  Lava 
Flow  aa' 


Scale  1  inch  =  1COO  feet 

FIG.  199.   Measurement  of  the  Thickness 
of  Inclined  Strata 


11.  Which  is  the  older  fault,  in  Figure  198,  —/or/'?  When  did  the 
lava  flow  occur?    How  long  a  time  elapsed  between  the  formation  of  the 
two  faults  as  measured  in  the  work  done  in  the  interval?    How  long  a 
time  since  the  formation  of  the  later  fault? 

12.  Measure  by  the  scale  the  thickness  be  of  the  coal-bearing  strata 
outcropping  from  a  to  b  in  Figure  199.    On  any  convenient  scale  draw 
a  similar  section  of  strata  with  a  dip  of  30°  outcropping  along  a  hori- 
zontal line  normal  to  the  strike  one  thousand  feet  in  length,  and  meas- 
ure the  thickness  of  the  strata  by  the  scale  employed.    The  thickness 
may  also  be  calculated  by  trigonometry. 


UNCONFORMITY 

Strata  deposited  one  upon  another  in  an  unbroken  succession 
are  said  to  be  conformable.  But  the  continuous  deposition  of 
strata  is  often  interrupted  by  moventents  of  the  earth's  crust. 
Old  sea  floors  are  lifted  to 
form  land  and  are  again 
depressed  beneath  the  sea 
to  receive  a  cover  of  sedi- 
ments only  after  an  inter- 
val during  which  they  FIG.  200.  Unconformity  between 

Parallel  Strata 
were  carved  by  subaenal 

erosion.  An  erosion  surface  which  thus  parts  older  from  youngei 
strata  is  known  as  an  unconformity,  and  the  strata  above  it  are 


228 


THE  ELEMENTS  OF  GEOLOGY 


said  to  be  unconformable  with  the  rocks  below,  or  to  rest  uncon- 
formably  upon  them.  An  unconformity  thus  records  movements 
of  the  crust  and  a  consequent  break  in  the  deposition  of  the  strata. 
It  denotes  a  period  of  land  erosion  of  greater  or  less  length, 
which  may  sometimes  be  roughly  measured  by  the  stage  in  the 

erosion  cycle  which  the 
land  surface  had  attained 
before  its  burial.  Uncon- 
formable strata  may  be 
parallel,  as  in  Figure  200, 


FIG.  201.   Unconformity  between  Non- 
parallel  Strata 


where  the  record  includes 
the  deposition  of  strata  a, 

their  emergence,  the  erosion  of  the  land  surface  ss,  a  submer- 

gence and  the  deposit  of  the  strata  I,  and  lastly,  emergence  and 

the  erosion  of  the  present  surface  s's'. 

Often  the  earth  movements  to  which  the  uplift  or  depression 

was  due  involved  tilting  or  folding  of  the  earlier  strata,  so  that 

the  strata  are  now  nonparallel  as  well  as  unconformable.    In 

Figure  201,  for  example,  the  record  includes  deposition,  '  uplift, 

and  tilting  of  a\  erosion,  depression,  the  deposit  of  fr;  and  finally 

the    uplift   which    has 

brought    the    rocks    to 

open  air  and  permitted 

the  dissection  by  which 

the  unconformity  is  re- 

vealed. 

From   this    section    we 

infer    that    during    early 

Silurian    times    the    area 

was    sea,    and    thick    sea 

muds  were    laid  upon   it. 

These  were   later    altered 

to  hard  slates  by  pressure 
and  upfolded  into  moun- 

tains.      During   the   later 


G  202.  Carboniferous  Limestones  resting 
unconformably  on  Early  Silurian  Slates, 
Yorkshire,  England 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


229 


Silurian  and  the  Devonian  the  area  was  land  and  suffered  vast  denuda- 
tion. In  the  Carboniferous  period  it  was  lowered  beneath  the  sea  and 
received  a  cover  of  limestone. 

The  age  of  mountains.  It  is  largely  by  means  of  unconformi- 
ties that  we  read  the  history  of  mountain  making  and  other 
deformations  and  movements  of  the 
crust.  In  Figure  203,  for  example, 
the  deformation  which  upfolded  the 
range  of  mountains  took  place  after 
the  deposit  of  the  series  of  strata  a 
of  which  the  mountains  are  corn- 


FIG.  203.  Diagram  illustrating 
how  the  Age  of  Mountains 
is  determined 


posed,  and  before  the  deposit  of  the  stratified  rocks  b,  which 
rest  unconformably  on  a  and  have  not  shared  their  uplift. 

Most  great  mountain  ranges,  like  the  Sierra  Nevada  and  the 
Alps,  mark  lines  of  weakness  along  which  the  earth's  crust  has 
yielded  again  and  again  during  the  long  ages  of  geological  time. 

The  strata  deposited  at 
various  times  about 
their  flanks  have  been 
infolded  by  later  crump- 
lings  with  the  original 


FIG.  204.   Section  of  Mountain  Range  showing 
Repeated  Uplifts 

a,  strata  whose  folding  formed  a  mountain  range ; 
uu,  baseleveled  surface  produced  by  long  de- 
nudation of  the  mountains;  b,  tilted  strata 
resting  unconformably  on  a;  c,  horizontal 
strata  parted  from  6  by  the  unconformity 
u'u'.  The  first  uplift  of  the  range  preceded 
the  period  of  time  when  6  was  deposited.  The 
second  uplift,  to  which  the  present  mountains 
owe  their  height,  was  later  than  this  period  but 
earlier  than  the  period  when  strata  c  were  laid 


mountain  mass,  and 
have  been  repeatedly 
crushed,  inverted, 
faulted,  intruded  with 
igneous  rocks,  and  de- 
nuded. The  structure 
of  great  mountain 
ranges  thus  becomes 


exceedingly  complex 
and  difficult  to  read.  A  comparatively  simple  case  of  repeated 
uplift  is  shown  in  Figure  204.  In  the  section  of  a  portion  of 
the  Alps  shown  in  Figure  179  a  far  more  complicated  history 
may  be  deciphered. 


230 


THE  ELEMENTS  OF  GEOLOGY 


FIG.  205.   Unconformity  showing  Buried  Valleys 

Im,  limestone;  sh,  shale;  r,  r',  and  r",  river  silts  filling  eroded  valleys  in  the  lime- 
stone. The  upper  surface  of  the  limestone  is  evidently  a  land  surface  devel- 
oped by  erosion.  The  valleys  which  trench  it  are  narrow  and  steep-sided; 
hence  the  land  surface  had  not  reached  maturity.  The  sands  and  muds,  now 
hardened  to  firm  rock,  which  fill  these  valleys,  r,  r',  and  r",  contain  no  relics 
of  the  sea,  but  instead  the  remains  of  land  animals  and  plants.  They  are 
river  deposits,  and  we  may  infer  that  owing  to  a  subsidence  the  young  rivers 
ceased  to  degrade  their  channels  and  slowly  filled  their  gorges  with  sands  and 
silts.  The  overlying  shale  records  a  further  depression  which  brought  the 
land  below  the  level  of  the  sea.  A  section  similar  to  this  is  to  be  seen  in  the 
coal  mines  of  Bernissant,  Belgium,  where  a  gorge  twice  as  deep  as  that  of 
Niagara  was  discovered,  within  whose  ancient  river  deposits  were  found  en- 
tombed the  skeletons  of  more  than  a  score  of  the  huge  reptiles  characteristic 
of  the  age  when  the  gorge  was  cut  and  filled 


FIG.  206.   Unconformity  showing  Buried  Mountains,  Scotland 

ffn,  ancient  crystalline  rocks ;  ss,  marine  sandstones.  The  surface  bb  of  the  an- 
cient crystalline  rocks  is  mountainous,  with  peaks  rising  to  a  height  of  as 
much  as  three  thousand  feet.  It  is  one  of  the  most  ancient  land  surfaces  on 
the  planet  and  is  covered  unconformably  with  pre-Cambrian  sandstones  thou- 
sands of  feet  in  thickness,  in  which  the  Torridonian  Mountains  of  Scotland  have 
been  carved.  What  has  been  the  history  of  the  region  since  the  mountainous 
surface  bb  was  produced  by  erosion  ? 

Unconformities  in  the  Colorado  Canyon,  Arizona.  How  geological  his- 
tory may  be  read  in  unconformities  is  further  illustrated  in  Figures 
207  and  208.  The  dark  crystalline  rocks  a  at  the  bottom  of  the  can- 
yon are  among  the  most  ancient  known,  and  are  overlain  unconformably 
by  a  mass  of  tilted  coarse  marine  sandstones  6,  whose  total  thickness 
is  not  seen  in  the  diagram  and  measures  twelve  thousand  feet  perpen- 
dicularly to  the  dip.  Both  a  and  b  rise  to  a  common  level  rm',  and  upon 
them  rest  the  horizontal  sea-laid  strata  c,  in  which  the  upper  portion 
of  the  canyon  has  been  cut. 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


231 


Note  that  the  crystalline  rocks  a  have  been  crumpled  and  crushed. 
Comparing  their  structure  with  that  of  folded  mountains,  what  do  you 
infer  as  to  their  relief  after  their  deformation  ?  To  which  surface  were 


FIG.  207.   Diagram  of  the  Wall  of  the  Colorado  Canyon,  Arizona, 
showing  Unconformities 

they  first  worn  down,  mm'  or  wn?  Describe  and  account  for  the  sur- 
face mm'.  How  does  it  differ  from  the  surface  of  the  crystalline  rocks 
seen  in  the  Torridonian  Mountains  (Fig.  206),  and  why?  This  sur- 
face mm'  is  one  of  the  oldest  land  surfaces  of  which  any  vestige  remains. 


^_:_~—  c 

^rC^j-  -n_~-     ••  w 


FIG.  208.  View  of  the  North  Wall  of  the  Grand  Canyon  of  the  Colorado 
River,  Arizona,  showing  the  Unconformities  illustrated  in  Figure  207 

It  is  a  bit  of  fossil  geography  buried  from  view  since  the  earliest  geo- 
logical ages  and  recently  brought  to  light  by  the  erosion  of  the  canyon. 
How  did  the  surface  mm'  come  to  receive  its  cover  of  sandstones  b  ? 
From  the  thickness  and  coarseness  of  these  sediments  draw  inferences 
as  to  the  land  mass  from  which  they  were  derived.  Was  it  rising 


232  THE  ELEMENTS  OF  GEOLOGY 

or  subsiding?  high  or  low?  Were  its  streams  slow  or  swift?  Was  the 
amount  of  erosion  small  or  great? 

Note  the  strong  dip  of  these  sandstones  b.  Was  the  surface  mm' 
tilted  as  now  when  the  sandstones  were  deposited  upon  it  ?  When  was 
it  tilted  ?  Draw  a  diagram  showing  the  attitude  of  the  rocks  after  this 
tilting  occurred,  and  their  height  relative  to  sea  level. 

The  surface  nn'  is  remarkably  even,  although  diversified  by  some  low 
hills  which  rise  into  the  bedded  rocks  of  c,  and  it  may  be  traced  for 
long  distances  up  and  down  the  canyon.  AVere  the  layers  of  b  and  the 
surface  mm'  always  thus  cut  short  by  nn'  as  now  ?  What  has  made  the 
surface  nn'  so  even  ?  How  does  it  come  to  cross  the  hard  crystalline 
rocks  a  and  the  weaker  sandstones  b  at  the  same  impartial  level? 
How  did  the  sediments  of  c  come  to  be  laid  upon  it?  Give  now  the 
entire  history  recorded  in  the  section,  and  in  addition  that  involved  in 
the  production  of  the  platform  P,  shown  in  Figure  130,  and  that  of  the 
cutting  of  the  canyon.  How  does  the  time  involved  in  the  cutting 
of  the  canyon  compare  with  that  required  for  the  production  of  the 
surfaces  mm',  nn',  and  P? 


FIG.  209.   Unconformity  between  the  Cambrian  and  Pre-Cambrian  Rocks, 

Wisconsin 

z,  pre-Carabrian  rocks,  igneous  and  metamorphic,  greatly  deformed ;  a',  zone 
of  decomposed  pre-Carabrian  rocks  and  residual  clays  on  which  rest  the 
Cambrian  sandstones  b.  What  unconformity  do  you  find  here?  What  two 
peneplains  do  you  discover?  Which  is  the  older?  Which  was  the  more  com- 
plete? To  what  stage  has  the  present  erosion  cycle  advanced?  Suggest  a 
reason  why  the  valleys  in  the  Cambrian  are  wider  than  those  in  the  pre- 
Cambrian.  When  did  the  decay  of  the  pre-Cambrian  rocks  of  zone  a'  take 
place,  and  under  what  circumstances?  Give  the  entire  history  recorded  in 
this  section,  stating  the  successive  cycles  of  erosion  in  their  order  and  the 
causes  which  brought  each  cycle  to  a  close 


CHAPTER   X 
EARTHQUAKES 

Any  sudden  movement  of  the  rocks  of  the  crust,  as  when 
they  tear  apart  when  a  fissure  is  formed  or  extended,  or  slip 
from  tune  to  time  along  a  growing  fault,  produces  a  jar  called 
an  earthquake,  which  spreads  in  all  directions  from  the  place 
of  disturbance. 

The  Charleston  earthquake.  On  the  evening  of  August  31,  1886,  the 
city  of  Charleston,  S.  C.,  was  shaken  by  one  of  the  greatest  earthquakes 
which  has  occurred  in  the  United  States.  A  slight  tremor  which 
rattled  the  windows  was  followed  a  few  seconds  later  by  a  roar,  as  of 
subterranean  thunder,  as  the  main  shock  passed  beneath  the  city. 
Houses  swayed  to  and  fro,  and  their  heaving  floors  overturned  furniture 
and  threw  persons  off  their  feet  as,  dizzy  and  nauseated,  they  rushed 
to  the  doors  for  safety.  In  sixty  seconds  a  number  of  houses  were  com- 
pletely wrecked,  fourteen  thousand  chimneys  were  toppled  over,  and  in 
all  the  city  scarcely  a  building  was  left  without  serious  injury.  In  the 
vicinity  of  Charleston  railways  were  twisted  and  trains  derailed.  Fis- 
sures opened  in  the  loose  superficial  deposits,  and  in  places  spouted 
water  mingled  with  sand  from  shallow  underlying  aquifers. 

The  point  of  origin,  or  focus,  of  the  earthquake  was  inferred  from 
subsequent  investigations  to  be  a  rent  in  the  rocks  about  twelve  miles 
beneath  the  surface.  From  the  center  of  greatest  disturbance,  which 
lay  above  the  focus,  a  few  miles  northwest  of  the  city,  the  surface 
shock  traveled  outward  in  every  direction,  with  decreasing  effects,  at  the 
rate  of  nearly  two  hundred  miles  per  minute.  It  was  felt  from  Boston 
to  Cuba,  and  from  eastern  Iowa  to  the  Bermudas,  over  a  circular  area 
whose  diameter  was  a  thousand  miles. 

An  earthquake  is  transmitted  from  the  focus  through  the 
elastic  rocks  of  the  crust,  as  a  wave,  or  series  of  waves,  of  com- 
pression and  rarefaction,  much  as  a  sound  wave  is  transmitted 


234 


THE  ELEMENTS  OF  GEOLOGY 


through  the  elastic  medium  of  the  air.  Each  earth  particle 
vibrates  with  exceeding  swiftness,  but  over  a  very  short  path. 
The  swing  of  a  particle  in  firm  rock  seldom  exceeds  one  tenth 
of  an  inch  in  ordinary  earthquakes,  and  when  it  reaches  one 
half  an  inch  and  an  inch,  the  movement  becomes  dangerous 
and  destructive. 

The  velocity  of  earthquake  waves,  like  that  of  all  elastic 
waves,  varies  with  the  temperature  and  elasticity  of  the  medium. 
In  the  deep,  hot,  elastic  rocks  they  speed  faster  than  in  the 


FIG.  210.    Block  of  the  Earth's  Crust  shaken  by  an  Earthquake 

x,  focus;  a,  b,c,d,  successive  spheroidal  waves  in  the  crust;  a',  &',  c',  d',  succes- 
sive surface  waves  produced  by  the  outcropping  of  a,  b,  c,  and  d 

cold  and  broken  rocks  near  the  surface.  The  deeper  the  point 
of  origin  and  the  more  violent  the  initial  shock,  the  faster  and 
farther  do  the  vibrations  run. 

Great  earthquakes,  caused  by  some  sudden  displacement  or 
some  violent  rending  of  the  rocks,  shake  the  entire  planet. 
Their  waves  run  through  the  body  of  the  earth  at  the  rate  of 
about  three  hundred  and  fifty  miles  a  minute,  and  more  slowly 
round  its  circumference,  registering  their  arrival  at  opposite 
sides  of  the  globe  on  the  exceedingly  delicate  instruments  of 
modern  earthquake  observatories. 

Geological  effects.  Even  great  earthquakes  seldom  produce 
geological  effects  of  much  importance.  Landslides  may  be 
shaken  down  from  the  sides  of  mountains  and  hills,  and 
cracks  may  be  opened  in  the  surface  deposits  of  plains  ;  but 


EARTHQUAKES  235 

the  transient  shiver,  which  may  overturn  cities  and  destroy 
thousands  of  human  lives,  runs  through  the  crust  and  leaves 
it  much  the  same  as  before. 

Earthquakes  attending  great  displacements.  Great  earth- 
quakes frequently  attend  the  displacement  of  large  masses  of 
the  rocks  of  the  crust.  In  1822  the  coast  of  Chile  was  sud- 
denly raised  three  or  four  feet,  and  the  rise  was  five  or  six  feet 
a  mile  inland.  In  1835  the  same  region  was  again  upheaved 
from  two  to  ten  feet.  In  each  instance  a  destructive  earthquake 
was  felt  for  one  thousand  miles  along  the  coast. 

The  great  California  earthquake  of  1906.  A  sudden  dislocation  occurred 
in  1906  along  an  ancient  fault  plane  which  extends  for  300  miles  through 
western  California.  The  vertical  displacement  did  not  exceed  four 
feet,  while  the  horizontal  shifting  reached  a  maximum  of  twenty  feet. 
Fences,  rows  of  trees,  and  roads  which  crossed  the  fault  wrere  broken 
and  offset.  The  latitude  and  longitude  of  all  points  over  thousands  of 
square  miles  were  changed.  On  each  side  of  the  fault  the  earth  blocks 
moved  in  opposite  directions,  the  block  on  the  east  moving  southward  and 
that  on  the  west  moving  northward  and  to  twice  the  distance.  East  and 
west  of  the  fault  the  movements  lessened  with  increasing  distance  from  it. 

This  sudden  slip  set  up  an  earthquake  lasting  sixty-five  seconds, 
followed  by  minor  shocks  recurring  for  many  days.  In  places  the  jar 
shook  down  the  waste  on  steep  hillsides,  snapped  off  or  uprooted  trees, 
and  rocked  houses  from  their  foundations  or  threw  down  their  walls  or 
chimneys.  The  water  mains  of  San  Francisco  were  broken,  and  the 
city  was  thus  left  defenseless  against  a  conflagration  which  destroyed 
1500,000,000  worth  of  property.  The  destructive  effects  varied  with 
the  nature  of  the  ground.  Buildings  on  firm  rock  suffered  least,  while 
those  on  deep  alluvium  were  severely  shaken  by  the  undulations,  like 
water  waves,  into  which  the  loose  material  was  thrown.  Well-braced 
steel  structures,  even  of  the  largest  size,  were  earthquake  proof,  and 
buildings  of  other  materials,  when  honestly  built  and  intelligently 
designed  to  withstand  earthquake  shocks,  usually  suffered  little  injury. 
The  length  of  the  intervals  between  severe  earthquakes  in  western 
California  shows  that  a  great  dislocation  so  relieves  the  stresses  of  the 
adjacent  earth  blocks  that  scores  of  years  may  elapse  before  the  stresses 
again  accumulate  and  cause  another  dislocation. 


236          THE  ELEMENTS  OF  GEOLOGY 

Perhaps  the  most  violent  earthquake  which  ever  visited  the  United 
States  attended  the  depression,  in  1812,  of  a  region  seventy-five  miles 
long  and  thirty  miles  wide,  near  New  Madrid,  Mo.  Much  of  the  area 
was  converted  into  swamps  and  some  into  shallow  lakes,  while  a  region 
twenty  miles  in  diameter  was  bulged  up  athwart  the  channel  of  the 
Mississippi.  Slight  quakes  are  still  felt  in  this  region  from  time  to 
time,  showing  that  the  strains  to  which  the  dislocation  was  due  have 
not  yet  been  fully  relieved. 

Earthquakes  originating  beneath  the  sea.  Many  earthquakes 
originate  beneath  the  sea,  and  in  a  number  of  examples  they 
seem  to  have  been  accompanied,  as  soundings  indicate, -by  local 
subsidences  of  the  ocean  bottom.  There  have  been  instances 
where  the  displacement  has  been  sufficient  to  set  the  entire 
Pacific  Ocean  pulsating  for  many  hours.  In  mid  ocean  the  wave 
thus  produced  has  a  height  of  only  a  few  feet,  while  it  may  be 
two  hundred  miles  in  width.  On  shores  near  the  point  of  ori- 
gin destructive  waves  two  or  three  score  feet  in  height  roll  in, 
and  on  coasts  thousands  of  miles  distant  the  expiring  undula- 
tions may  be  still  able  to  record  themselves  on  tidal  gauges. 

Distribution  of  earthquakes.  Every  half  hour  some  consid- 
erable area  of  the  earth's  surface  is  sensibly  shaken  by  an  earth- 
quake, but  earthquakes  are  by  no  means  uniformly  distributed 
over  the  globe.  As  we  might  infer  from  what  we  know  as  to 
their  causes,  earthquakes  are  most  frequent  in  regions  now 
.undergoing  deformation.  Such  are  young  rising  mountain  ranges, 
fault  lines  where  readjustments  recur  from  time  to  time,  and 
the  slopes  of  suboceanic  depressions  whose  steepness  suggests 
that  subsidence  may  there  be  in  progress. 

Earthquakes,  often  of  extreme  severity,  frequently  visit  the  lofty  and 
young  ranges  of  the  Andes,  while  they  are  little  known  in  the  subdued 
old  mountains  of  Brazil.  The  Highlands  of  Scotland  are  crossed  by  a 
deep  and  singularly  straight  depression  called  the  Great  Glen,  which 
has  been  excavated  along  a  very  ancient  line  of  dislocation.  The  earth- 
quakes which  occur  from  time  to  time  in  this  region^  such  as  the  Inver- 
ness earthquake  in  1891,  are  referred  to  slight  slips  along  this  fault  plane. 


EARTHQUAKES  237 

In  Japan,  earthquakes  are  very  frequent.  More  than  a  thousand 
are  recorded  every  year,  and  twenty-nine  world-shaking  earthquakes 
occurred  in  the  three  years  ending  with  1901.  They  originate,  for  the 
most  part,  well  down  on  the  eastern  flank  of  the  earth  fold  whose  sum- 
mit is  the  mountainous  crest  of  the  islands,  and  which  plunges  steeply 
beneath  the  sea  to  the  abyss  of  the  Tuscarora  Deep. 

Minor  causes  of  earthquakes.  Since  any  concussion  with- 
in the  crust  sets  up  an  earth  jar,  there  are  several  minor  causes 
of  earthquakes,  such  as  volcanic  explosions  and  even  the  col- 
lapse of  the  roofs  of  caves.  The  earthquakes  which  attend  the 
eruption  of  volcanoes  are  local,  even  in  the  case  of  the  most 
violent  volcanic  paroxysms  known.  When  the  top  of  a  volcano 
has  been  blown  to  fragments,  the  accompanying  earth  shock  has 
sometimes  not  been  felt  more  than  twenty-five  miles  away. 

Depth  of  focus.  The  focus  of  the  Charleston  earthquake, 
estimated  at  about  twelve  miles  below  the  surface,  was  excep- 
tionally deep.  Volcanic  earthquakes  are  particularly  shallow, 
and  probably  no  earthquakes  known  have  started  at  a  greater 
depth  than  fifteen  or  twenty  miles.  This  distance  is  so  slight 
compared  with  the  earth's  radius  that  we  may  say  that  earth- 
quakes are  but  skin-deep. 

Should  you  expect  the  velocity  of  an  earthquake  to  be  greater  in  a 
peneplain  or  in  a  river  delta  V 

After  an  earthquake,  piles  on  which  buildings  rested  were  found 
driven  into  the  ground,  and  chimneys  crushed  at  base.  From  what 
direction  did  the  shock  come  ? 

Chimneys  standing  on  the  south  walls  of  houses  toppled  over  on  the 
roof.  Should  you  infer  that  the  shock  in  this  case  came  from  the  north 
or  south  ? 

How  should  you  expect  a  shock  from  the  east  to  affect  pictures  hang- 
ing on  the  east  and  the  west  walls  of  a  room  ?  how  the  pictures  hanging 
on  the  north  and  the  south  walls  ? 

In  parts  of  the  country,  as  in  southwestern  Wisconsin,  slender 
erosion  pillars,  or  "  monuments,"  are  common.  What  inference  could 
you  draw  as  to  the  occurrence  in  such  regions  of  severe  earthquakes  in 
the  recent  past  ? 


CHAPTER   XI 
VOLCANOES 

Connected  with  movements  of  the  earth's  crust  which  take 

place  so  slowly  that  they  can  be  inferred  only  from  their  effects 

is  one  of  the  most  rapid  and  impressive  of  all  geological  processes, 

—  the  extrusion  of  molten  rock  from  beneath  the  surface  of  the 

earth,  giving  rise  to  all  the  various  phenomena  of  volcanoes. 

In  a  volcano,  molten  rock  from  a  region  deep  below,  which 
we  may  call  its  reservoir,  ascends  through  a  pipe  or  fissure  to 
the  surface.  The  materials  erupted  may  be  spread  over  vast 
areas,  or,  as  is  commonly  the  case,  may  accumulate  about  the 
opening,  forming  a  conical  pile  known  as  the  volcanic  cone.  It 
is  to  this  cone  that  popular  usage  refers  the  word  volcano ;  but 
the  cone  is  simply  a  conspicuous  part  of  the  volcanic  mechanism 
whose  still  more  important  parts,  the  reservoir  and  the  pipe,  are 
hidden  from  view. 

Volcanic  eruptions  are  of  two  types,  —  effusive  eruptions, 
in  which  molten  rock  wells  up  from  below  and  flows  forth  in 
streams  of  lava  (a  comprehensive  term  applied  to  all  kinds  of 
rock  emitted  from  volcanoes  in  a  molten  state),  and  explosive 
eruptions,  in  which  the  rock  is  blown  out  in  fragments  great 
and  small  by  the  expansive  force  of  steam. 

ERUPTIONS  OF  THE  EFFUSIVE  TYPE 

The  Hawaiian  volcanoes.  The  Hawaiian  Islands  are  all  vol- 
canic in  origin,  and  have  a  linear  arrangement  characteristic  of 
many  volcanic  groups  in  all  parts  of  the  world.  They  are  strung 
along  a  northwest-southeast  line,  their  volcanoes  standing  in 

238 


VOLCANOES 


239 


two  parallel  rows  as  if  reared  along  two  adjacent  lines  of  frac- 
ture or  folding.  In  the  northwestern  islands  the  volcanoes 
have  long  been  extinct  and  are  worn  low  by  erosion.  In  the 
southeastern  island, 
Hawaii,  three  volca- 
noes are  still  active  and 
in  process  of  building. 
Of  these  Mauna  Loa, 
the  monarch  of  vol- 
canoes, with  a  girth  of 
two  hundred  miles  and 
a  height  of  nearly  four- 
teen thousand  feet  above 
sea  level,  is  a  lava  dome 
the  slope  of  whose  sides  does  not  average  more  than  five 
degrees.  On  the  summit  is  an  elliptical  basin  ten  miles  in  cir- 
cumference and  several  hundred  feet  deep.  Concentric  cracks 
surround  the  rim,  and  from  time  to  time  the  basin  is  enlarged 
as  great  slices  are  detached  from  the  vertical  walls  and  engulfed. 


FIG.  211.   Mauna  Loa 


FIG.  212.   Caldera  of  Kilauea 

Such  a  volcanic  basin,  formed  by  the  insinking  of  the  top  of 
the  cone,  is  called  a  caldera. 

On  the  flanks  of  Mauna  Loa,  four  thousand  feet  above  sea  level,  lies 
the  caldera  of  Kilauea,  an  independent  volcano  whose  dome  has  been 
joined  to  the  larger  mountain  by  the  gradual  growth  of  the  two.  In 


240         THE  ELEMENTS  OF  GEOLOGY 

each  caldera  the  floor,  which  to  the  eye  is  a  plain  of  black  lava,  is  the 
congealed  surface  of  a  column  of  molten  rock.  At  times  of  an  eruption 
lakes  of  boiling  lava  appear  which  may  be  compared  to  air  holes  in  a 
frozen  river.  Great  waves  surge  up,  lifting  tons  of  the  fiery  liquid  a 
score  of  feet  in  air,  to  fall  back  with  a  mighty  plunge  and  roar,  and 
occasionally  the  lava  rises  several  hundred  feet  in  fountains  of  dazzling 
brightness.  The  lava  lakes  may  flood  the  floor  of  the  basin,  but  in 


FIG.  213.   Portion  of  the  Caldera  of  Kilauea  after  a  Collapse 
following  an  Eruption 

historic  times  have  never  been  known  to  fill  it  and  overflow  the  rim. 
Instead,  the  heavy  column  of  lava  breaks  way  through  the  sides  of  the 
mountain  and  discharges  in  streams  which  flow  down  the  mountain 
slopes  for  a  distance  sometimes  of  as  much  as  thirty-five  miles.  With 
the  drawing  off  of  the  lava  the  column  in  the  duct  of  the  volcano 
lowers,  and  the  floor  of  the  caldera  wholly  or  in  part  subsides.  A  black 
and  steaming  abyss  marks  the  place  of  the  lava  lakes  (Fig.  213).  After 
a  time  the  lava  rises  in  the  duct,  the  floor  is  floated  higher,  and  the  boil- 
ing lakes  reappear. 


VOLCANOES  241 

The  eruptions  of  the  Hawaiian  volcanoes  are  thus  of  the 
effusive  type.  The  column  of  lava  rises,  breaks  through  the 
side  of  the  mountain,  and  discharges  in  lava  streams.  There 
are  no  explosions,  and  usually  no  earthquakes,  or  very  slight 
ones,  accompany  the  eruptions.  The  lava  in  the  calderas  boils 
because  of  escaping  steam,  but  the  vapor  emitted  is  compara- 
tively little,  and  seldom  hangs  above  the  summits  in  heavy 
clouds.  We  see  here  in  its  simplest  form  the  most  impressive 
and  important  fact  in  all  volcanic  action,  —  molten  rock  has  been 
driven  upward  to  the  surface  from  some  deep-lying  source. 

Lava  flows.  As  lava  issues  from  the  side  of  a  volcano  or 
overflows  from  the  summit,  it  flows  away  in  a  glowing  stream 


FIG.  214.  Pahoehoe  Lava,  Hawaii 

resembling  molten  iron  drawn  white-hot  from  an  iron  furnace. 
The  surface  of  the  stream  soon  cools  and  blackens,  and  the 
hard  crust  of  nonconducting  rock  may  grow  thick  and  firm 
enough  to  form  a  tunnel,  within  which  the  fluid  lava  may  flow 
far  before  it  loses  its  heat  to  any  marked  degree.  Such  tun- 
nels may  at  last  be  left  as  caves  by  the  draining  away  of  the 
lava,  and  are  sometimes  several  miles  in  length. 

Pahoehoe  and  aa.  When  the  crust  of  highly  fluid  lava  remains 
unbroken  after  its  first  freezing,  it  presents  a  smooth,  hummocky,  and 
ropy  surface  known  by  the  Hawaiian  term  pdhoehoe  (Fig.  214).  On  the 


242         THE  ELEMENTS  OF  GEOLOGY 

other  hand,  the  crust  of  a  viscid  flow  may  be  broken  and  splintered  as  it 
is  dragged  along  by  the  slowly  moving  mass  beneath.  The  stream  then 
appears  as  a  field  of  stones  clanking  and  grinding  on,  with  here  and 
there  from  some  chink  a  dull  red  glow  or  a  wisp  of  steam.  It  sets  to  a 
surface  called  aa,  of  broken,  sharp-edged  blocks,  which  is  often  both 
difficult  and  dangerous  to  traverse  (Fig.  215). 


FIG.  215.   Lava  Flow  of  the  Aa  Type  ;  Cinder  Cones  in  the 
Distance,  Arizona 

Fissure  eruptions.  Some  of  the  largest  and  most  important 
outflows  of  lava  have  not  been  connected  with  volcanic  cones, 
but  have  been  discharged  from  fissures,  flooding  the  country 
far  and  wide  with  molten  rock.  Sheet  after  sheet  of  molten 
rock  has  been  successively  outpoured,  and  there  have  been 
built  up,  layer  upon  layer,  plateaus  of  lava  thousands  of  feet 
in  thickness  and  many  thousands  of  square  miles  in  area. 

Iceland.  This  island  plateau  has  been  rent  from  time  to  time  by 
fissures  from  which  floods  of  lava  have  outpoured.  In  some  instances 
the  lava  discharges  along  the  whole  length  of  the  fissure,  but  more 
often  only  at  certain  points  upon  it.  The  Laki  fissure,  twenty  miles 
long,  was  in  eruption  in  1783  for  seven  months.  The  inundation  of 


VOLCANOES 


243 


fluid  rock  which  poured  from  it  is  the  largest  of  historic  record,  reach- 
ing a  distance  of  forty-seven  miles  and  covering  two  hundred  and 
twenty  square  miles  to  an  average  depth  of  a  hundred  feet.  At  the 
present  time  the  fissure  is  traced  by  a  line  of  several  hundred  insignifi- 
cant mounds  of  f  rag- 
mental  materials 
which  mark  where 
the  lava  issued 
(Fig.  216). 

The  distance  to 
which  the  fissure 
eruptions  of  Iceland 
flow  on  slopes  ex- 
tremely gentle  is 
noteworthy.  One 
such  stream  is  ninety 
miles  in  length,  and 
another  seventy 
miles  long  has  a 
slope  of  little  more 
than  one  half  a  de- 


FIG.  216.  Small  Cinder  Cones  marking  an  Eruptive 
Fissure,  Iceland 


gree. 

Where  1  ava  is 

emitted  at  one  point  and  flows  to  a  less  distance  there  is  gradually  built 
up  a  dome  of  the  shape  of  an  inverted  saucer  with  an  immense  base  but 
comparatively  low.  Many  lava  domes  have  been  discovered  in  Iceland, 
although  from  their  exceedingly  gentle  slopes,  often  but  two  or  three 

degrees,  they  long  escaped  the 
notice  of  explorers. 

The  entire  plateau  of  Ice- 
land, a  region  as  large  as  Ohio, 
is  composed  of  volcanic  prod- 


FIG.  217.   Diagram  illustrating  the  Struc- 
ture of  a  Lava  Plateau  such  as  Iceland 


ucts, — for  the  most  part  of 
successive  sheets  of  lava  whose 
total  thickness  falls  little  short 
of  two  miles.  The  lava  sheets 


If,  lava  flows ;  d,  dikes 

exposed  to  view  were  outpoured  in  open  air  and  not  beneath  the  sea ; 
for  peat  bogs  and  old  forest  grounds  are  interbedded  with  them,  and 
the  fossil  plants  of  these  vegetable  deposits  prove  that  the  plateau  has 


244          THE  ELEMENTS  OF  GEOLOGY 

long  been  building  and  is  very  ancient.  On  the  steep  sea  cliffs  of  the 
island,  where  its  structure  is  exhibited,  the  sheets  of  lava  are  seen  to  be 
cut  with  many  dikes,  —  fissures  which  have  been  filled  by  molten  rock, — 
and  there  is  little  doubt  that  it  was  through  these  fissures  that  the 
lava  outwelled  in  successive  flows  which  spread  far  and  wide  over  the 
country  and  gradually  reared  the  enormous  pile  of  the  plateau. 


ERUPTIONS  OF  THE  EXPLOSIVE  TYPE 

In  the  majority  of  volcanoes  the  lava  which  rises  in  the  pipe 
is  at  least  in  part  blown  into  fragments  with  violent  explosions 
and  shot  into  the  air  together  with  vast  quantities  of  water 
vapor  and  various  gases.  The  finer  particles  into  which  the 
lava  is  exploded  are  called  volcanic  dust  or  volcanic  ashes,  and 
are  often  carried  long  distances  by  the  wind  before  they  settle 
to  the  earth.  The  coarser  fragments  fall  about  the  vent  and 
there  accumulate  in  a  steep,  conical,  volcanic  mountain.  As  suc- 
cessive explosions  keep  open  the  throat  of  the  pipe,  there  remains 
on  the  summit  a  cup-shaped  depression  called  the  crater. 

Stromboli.  To  study  the  nature  of  these  explosions  we  may  visit 
Stromboli,  a  low  volcano  built  chiefly  of  fragmental  materials,  which 
rises  from  the  sea  off  the  north  coast  of  Sicily  and  is  in  constant 
though  moderate  action. 

Over  the  summit  hangs  a  cloud  of  vapor  which  strikingly  resembles 
the  column  of  smoke  puffed  from  the  smokestack  of  a  locomotive,  in 
that  it  consists  of  globular  masses,  each  the  product  of  a  distinct 
explosion.  At  night  the  cloud  of  vapor  is  lighted  with  a  red  glow  at 
intervals  of  a  few  minutes,  like  the  glow  on  the  trail  of  smoke  behind 
the  locomotive  when  from  time  to  time  the  fire  box  is  opened.  Because 
of  this  intermittent  light  flashing  thousands  of  feet  above  the  sea, 
Stromboli  has  been  given  the  name  of  the  Lighthouse  of  the  Mediter- 
ranean. 

Looking  down  into  the  crater  of  the  volcano,  one  sees  a  viscid  lava 
slowly  seething.  The  agitation  gradually  increases.  A  great  bubble 
forms.  It  bursts  with  an  explosion  which  causes  the  walls  of  the 
crater  to  quiver  with  a  miniature  earthquake,  and  an  outrush  of  steam 


VOLCANOES  245 

carries  the  fragments  of  the  bubble  aloft  for  a  thousand  feet  to  fall 
into  the  crater  or  on  the  mountain  side  about  it.  With  the  explosion 
the  cooled  and  darkened  crust  of  the  lava  is  removed,  and  the  light 
of  the  incandescent  liquid  beneath  is  reflected  from  the  cloud  of  vapor 
which  overhangs  the  cone. 

At  Stromboli  we  learn  the  lesson  that;  the  explosive  force  in 
volcanoes  is  that  of  steam.  The  lava  in  the  pipe  is  permeated 
with  it  much  as  is  a  thick  boiling  porridge.  The  steam  in  boil- 
ing porridge  is  unable  to  escape  freely  and  gathers  into  bubbles 
which  in  breaking  spurt  out  drops  of  the  pasty  substance;  in 
the  same  way  the  explosion  of  great  bubbles  of  steam  in  the 
viscid  lava  shoots  clots  and  fragments  of  it  into  the  air. 

Krakatoa.  The  most  violent  eruption  of  history,  that  of  Krakatoa,  a 
small  volcanic  island  in  the  strait  between  Sumatra  and  Java,  occurred 
in  the  last  week  of  August,  1883.  Continuous  explosions  shot  a  col- 
umn of  steam  and  ashes  seventeen  miles  in  air.  A  black  cloud,  beneath 
which  was  midnight  darkness  and  from  which  fell  a  rain  of  ashes 
and  stones,  overspread  the  surrounding  region  to  a  distance  of  one 
hundred  and  fifty  miles.  Launched  on  the  currents  of  the  upper  air, 
the  dust  was  swiftly  carried  westward  to  long  distances.  Three  days 
after  the  eruption  it  fell  on  the  deck  of  a  ship  sixteen  hundred  miles 
away,  and  in  thirteen  days  the  finest  impalpable  powder  from  the  vol- 
cano had  floated  round  the  globe.  For  many  months  the  dust  hung  over 
Europe  and  America  as  a  faint  lofty  haze  illuminated  at  sunrise  and 
sunset  with  brilliant  crimson.  In  countries  nearer  the  eruption,  as  in 
India  and  Africa,  the  haze  for  some  time  was  so  thick  that  it  colored 
sun  and  moon  with  blue,  green,  and  copper-red  tints  and  encircled 
them  with  coronas. 

At  a  distance  of  even  a  thousand  miles  the  detonations  of  the 
eruption  sounded  like  the  booming  of  heavy  guns  a  few  miles  away. 
In  one  direction  they  were  audible  for  a  distance  as  great  as  that  from 
San  Francisco  to  Cleveland.  The  entire  atmosphere  was  thrown  into 
undulations  under  which  all  barometers  rose  and  fell  as  the  air  waves 
thrice  encircled  the  earth.  The  shock  of  the  explosions  raised  sea 
waves  which  swept  round  the  adjacent  shores  at  a  height  of  more  than 
fifty  feet,  and  which  were  perceptible  halfway  around  the  globe. 


246         THE  ELEMENTS  OF  GEOLOGY 

At  the  close  of  the  eruption  it  was  found  that  half  the  mountain  had 
been  blown  away,  and  that  where  the  central  part  of  the  island  had 
been  the  sea  was  a  thousand  feet  deep. 

Martinique  and  St.  Vincent.  In  1902  two  dormant  volcanoes  of  the 
West  Indies,  Mt.  Pele"e  in  Martinique  and  Soufriere  in  St.  Vincent, 
broke  into  eruption  simultaneously.  No  lava  was  emitted,  but  there 
were  blown  into  the  air  great  quantities  of  ashes,  which  mantled  the 


FIG.  218.   Ruins  of  St.  Pierre,  Martinique  ;  Mt.  Pele"e  in  the  Distance 

adjacent  parts  of  the  islands  with  a  pall  as  of  gray  snow.  In  early  stages 
of  the  eruption  lakes  which  occupied  old  craters  were  discharged  and 
swept  down  the  ash-covered  mountain  valleys  in  torrents  of  boiling  mud. 
On  several  occasions  there  was  shot  from  the  crater  of  each  volcano 
a  thick  and  heavy  cloud  of  incandescent  ashes  and  steam,  which  rushed 
down  the  mountain  side  like  an  avalanche,  red  with  glowing  stones  and 
scintillating  with  lightning  flashes.  Forests  and  buildings  in  its  path 
were  leveled  as  by  a  tornado,  wood  was  charred  and  set  on  fire  by  the 
incandescent  fragments,  all  vegetation  was  destroyed,  and  to  breathe  the 


VOLCANOES 


247 


steam  and  hot,  suffocating  dust  of  the  cloud  was  death  to  every  living 
creature.  On  the  morning  of  the  8th  of  May,  1902,  the  first  of  these 
peculiar  avalanches  from  Mt.  Pele"e  fell  on  the  city  of  St.  Pierre  and 
instantly  destroyed  the  lives  of  its  thirty  thousand  inhabitants. 


FIG.  219.   An  Eruption  of  Vesuvius,  1872 

The  huge  column  of  dust  and  steam  rises  to  a  height  of  about  four  miles 
above  the  sea.  Drifting  down  the  wind,  the  vapor  condenses  into  copious 
rains.  Such  often  produce  destructive  torrents  of  mud  as  they  sweep  down 
the  ash-covered  mountain  side,  and  during  the  historic  eruption  of  Vesu- 
vius in  A.D.  79  the  city  of  Herculaneum  was  thus  buried.  Lava  flows  are 
marked  by  the  overhanging  clouds  of  aqueous  vapor  condensed  from  the 
steam  which  the  molten  rock  gives  off. 


The  eruptions  of  many  volcanoes  partake  of  both  the  effusive 
and  the  explosive  types :  the  molten  rock  in  the  pipe  is  in  part 


248          THE  ELEMENTS  OF  GEOLOGY 

blown  into  the  air  with  explosions  of  steam,  and  in  part  is  dis- 
charged in  streams  of  lava  over  the  lip  of  the  crater  and  from 
fissures  in  the  sides  of  the  cone.  Such  are  the  eruptions  of 
Vesuvius,  one  of  which  is  illustrated  in  Figure  219. 

Submarine  eruptions.  The  many  volcanic  islands  of  the  ocean 
and  the  coral  islands  resting  on  submerged  volcanic  peaks  prove 
that  eruptions  have  often  taken  place  upon  the  ocean  floor  and 
have  there  built  up  enormous  piles  of  volcanic  fragments  and 
lava.  The  Hawaiian  volcanoes  rise  from  a  depth  of  eighteen  thou- 
sand feet  of  water  and  lift  their  heads  to  about  thirty  thousand 
feet  above  the  ocean  bed.  Christmas  Island  (see  p.  194),  built 
wholly  beneath  the  ocean,  is  a  coral-capped  volcanic  peak,  whose 
total  height,  as  measured  from  the  bottom  of  the  sea,  is  more 
than  fifteen  thousand  feet.  Deep-sea  soundings  have  revealed 
the  presence  of  numerous  peaks  which  fail  to  reach  sea  level 
and  which  no  doubt  are  submarine  volcanoes.  A  number  of  vol- 
canoes on  the  land  were  submarine  in  their  early  stages,  as,  for 
example,  the  vast  pile  of  Etna,  the  celebrated  Sicilian  volcano, 
which  rests  on  stratified  volcanic  fragments  containing  marine 
shells  now  uplifted  from  the  sea. 

Submarine  outflows  of  lava  and  deposits  of  volcanic  frag- 
ments become  covered  with  sediments  during  the  long  intervals 
between  eruptions.  Such  volcanic  deposits  are  said  to  be  con- 
temporaneous, because  they  are  formed  during  the  same  period 
as  the  strata  among  which  they  are  imbedded.  Contempora- 
neous lava  sheets  may  be  expected  to  bake  the  surface  of  the 
stratum  on  which  they  rest,  while  the  sediments  deposited  upon 
them  are  unaltered  by  their  heat.  They  are  among  the  most 
permanent  records  of  volcanic  action,  far  outlasting  the  greatest 
volcanic  mountains  built  in  open  air. 

From  upraised  submarine  volcanoes,  such  as  Christmas  Island, 
it  is  learned  that  lava  flows  which  are  poured  out  upon  the 
bottom  of  the  sea  do  not  differ  materially  either  in  composition 
or  texture  from  those  of  the  land. 


VOLCANOES 


249 


VOLCANIC  PRODUCTS 

Vast  amounts  of  steam  are,  as  we  have  seen,  emitted  from  vol- 
canoes, and  comparatively  small  quantities  of  other  vapors,  such 
as  various  acid  and  sulphurous  gases.  The  rocks  erupted  from 
volcanoes  differ  widely  in  chemical  composition  and  in  texture. 

Acidic  and  basic  lavas.  Two  classes  of  volcanic  rocks  may 
be  distinguished,  —  those  containing  a  large  proportion  of  silica 


FIG.  220.   Cellular  Lava 

(silicic  acid,  Si02)  and  therefore  called  acidic,  and  those  contain- 
ing less  silica  and  a  larger  proportion  of  the  bases  (lime,  magnesia, 
soda,  etc.)  and  therefore  called  basic.  The  acidic  lavas,  of  which 
rhyolite  and  trachyte  are  examples,  are  comparatively  light  in 
color  and  weight,  and  are  difficult  to  melt.  The  basic  lavas,  of 
which  basalt  is  a  type,  are  dark  and  heavy  and  melt  at  a  lower 
temperature. 


260 


THE  ELEMENTS  OF  GEOLOGY 


Scoria  and  pumice.  The  texture  of  volcanic  rocks  depends  in 
part  011  the  degree  to  which  they  were  distended  by  the  steam 
which  permeated  them  when  in  a  molten  state.  They  harden 
into  compact  rock  where  the  steam  cannot  expand.  Where  the 
steam  is  released  from  pressure,  as  on  the  surface  of  a  lava 
stream,  it  forms  hubbies  (steam  blebs)  of  various  sizes,  which 
give  the  hardened  rock  a  cellular  structure  (Fig.  220).  In  this 


FIG.  221.   Amygdules  in  Lava 

way  are  formed  the  rough  slags  and  clinkers  called  scoria,  which 
are  found  on  the  surface  of  flows  and  which  are  also  thrown 
out  as  clots  of  lava  in  explosive  eruptions. 

On  the  surface  of  the  seething  lava  in  the  throat  of  the  vol- 
cano there  gathers  a  rock  foam,  which,  when  hurled  into  the 
air,  is  cooled  and  falls  as  pumice,  —  a  spongy  gray  rock  so  light 
that  it  floats  on  water. 

Amygdules.  The  steam  blebs  of  lava  flows  are  often  drawn 
out  from  a  spherical  to  an  elliptical  form  resembling  that  of  an 


VOLCANOES 


251 


almond,  and  after  the  rock  has  cooled  these  cavities  are  grad- 

ually filled  with  minerals  deposited  from  solution  by  under- 

ground water.    From  their  shape  such  casts  are  called  amygdules 

(Greek,  amygdalon, 

an  almond).  Amyg- 

dules   are    com- 

monly composed  of 

silica.     Lavas   con- 

tain both  silica  and 

the  alkalies,  potash 

and  soda,  and  after 

dissolving  the  alka- 

lies, percolating 

water  is  able  to  take  FlG-  222'  Polished  Section  of  an  Agate 

silica  also  into  solution.    Most  agates  are  banded  amygdules  hi 

which  the  silica  has  been  laid  in  varicolored,  concentric  layers 

(Fig.  222). 

Glassy  and  stony  lavas.    Volcanic  rocks  differ  in  texture 

according  also  to  the  rate  at  which  they  have  solidified.    When 

rapidly  cooled,  as  on  the  surface  of 
a  lava  flow,  molten  rock  chills  to  a 
glass,  because  the  minerals  of  which 
it  is  composed  have  not  had  time  to 
separate  themselves  from  the  fused 
mixture  and  form  crystals.  Under 
slow  cooling,  as  in  the  interior  of 
the  flow,  it  becomes  a  stony  mass 
composed  of  crystals  set  in  a  glassy 
paste.  In  thin  slices  of  volcanic 
lasg  Qne  may  see  under  the  micro- 
...  »  ,  , 

SGOPe    the     beginnings     of     crystal 

growth  in  filaments  and  needles 
and  feathery  forms,  which  are  the  rudiments  of  the  crystals  of 
various  minerals. 


FIG.  223.  Microsection  show- 
ing  the  Beginnings  of  Crys- 
tal  Growth  in  Glassy  Lava 


252 


THE  ELEMENTS  OF  GEOLOGY 


Spherulites,  which  also  mark  the  first  changes  of  glassy  lavas  toward 
a  stony  condition,  are  little  balls  within  the  rock,  varying  from  micro- 
scopic size  to  several  inches  in  diameter, 
and  made  up  of  radiating  fibers. 

Perlitic  structure,  common  among 
glassy  lavas,  consists  of  microscopic 
curving  and  interlacing  cracks,  due  to 
contraction. 


FIG.  224.   Perlitic  Structure 
and  Spherulites,  a,  a 


Flow  lines  are  exhibited  by  vol- 
canic rocks  both  to  the  naked  eye 
and  under  the  microscope.  Steam 
blebs,  together  with  crystals  and 

their  embryonic  forms,  are  left  arranged  in  lines  and  streaks 

by  the  currents  of  the  flowing  lava  as  it  stiffened  into  rock. 

Porphyritic  structure.    Rocks  whose  ground  mass  has  scat- 
tered through  it  large  conspicuous  crystals  (Fig.  226)  are  said 

to    be   porpliyritic, 

and  it  is  especially 

among    volcanic 

rocks    that    this 

structure   occurs. 

The  ground  mass  of 

porphyries  either 

may  be  glassy  or 

may  consist  in  part 

of  a  felt  of  minute 

crystals ;   in  either 

case  it  represents 

the  consolidation  of 

the  rock    after    its 

outpouring  upon 

the  surface.    On  the 

other  hand,  the  large  crystals  of  porphyry  have  slowly  formed 

deep  below  the  ground  at  an  earlier  date. 


VOLCANOES  253 

Columnar  structure.  Just  as  wet  starch  contracts  on  drying 
to  prismatic  forms,  so  lava  often  contracts  on  cooling  to  a  mass 
of  close-set,  prismatic,  and  commonly  six-sided  columns,  which 
stand  at  right  angles  to  the  cooling  surface.  The  upper  portion 
of  a  flow,  on  rapid  cooling  from  the  surface  exposed  to  the  air, 


FIG.  226.   Porphyritic  Structure 

may  contract  to  a  confused  mass  of  small  and  irregular  prisms ; 
while  the  remainder  forms  large  and  beautifully  regular  col- 
umns, which  have  grown  upward  by  slow  cooling  from  beneath 
(Fig.  227). 

FRAGMENTAL  MATERIALS 

Rocks  weighing  many  tons  are  often  thrown  from  a  volcano 
at  the  beginning  of  an  outburst  by  the  breaking  up  of  the  solid- 
ified floor  of  the  crater ;  and  during  the  progress  of  an  eruption 
large  blocks  may  be  torn  from  the  throat  of  the  volcano  by  the 
outrush  of  steam.  But  the  most  important  f ragmental  materials 
are  those  derived  from  the  lava  itself.  As  lava  rises  in  the  pipe, 
the  steam  which  permeates  it  is  released  from  pressure  and 


254 


VOLCANOES  255 

explodes,  hurling  the  lava  into  the  air  in  fragments  of  all  sizes, 
—  large  pieces  of  scoria,  lapilli  (fragments  the  size  of  a  pea  or 
walnut),  volcanic  "  sand,"  and  volcanic  "  ashes."  The  latter  resem- 
ble in  appearance  the  ashes  of  wood  or  coal,  but  they  are  not  in 
any  sense,  like  them,  a  residue  after  combustion. 

Volcanic  ashes  are  produced  in  several  ways :  lava  rising  in 
the  volcanic  duct  is  exploded  into  fine  dust  by  the  steam  which 
permeates  it;  glassy  lava,  hurled  into  the  air  and  cooled  sud- 
denly, is  brought  into  a  state  of  high  strain  and  tension,  and, 
like  Prince  Eupert's  drops,  flies  to  pieces  at  the  least  provocation. 
The  clash  of  rising  and  falling  projectiles  also  produces  some 
dust,  a  fair  sample  of  which  may  be  made  by  grating  together 
two  pieces  of  pumice. 

Beds  of  volcanic  ash  occur  widely  among  recent  deposits  in  the. 
western  United  States.  In  Nebraska  ash  beds  are  found  in  twenty 
counties,  and  are  often  as  white  as  powdered  pumice.  The  beds  grow 
thicker  and  coarser  toward  the  southwestern  part  of  the  state,  where 
their  thickness  sometimes  reaches  fifty  feet.  In  what  direction  would 
you  look  for  the  now  extinct  volcano  whose  explosive  eruptions  are  thus 
recorded  ? 

Tuff.  This  is  a  convenient  term  designating  any  rock  com- 
posed of  volcanic  fragments.  Coarse  tuffs  of  angular  fragments 
are  called  volcanic  breccia,  and  when  the  fragments  have  been 
rounded  and  sorted  by  water  the  rock  is  termed  a  volcanic  con- 
glomerate. Even  when  deposited  in  the  open  air,  as  on  the  slopes 
of  a  volcano,  tuffs  may  be  rudely  bedded  and  their  fragments 
more  or  less  rounded,  and  unless  marine  shells  or  the  remains 
of  land  plants  and  animals  are  found  as  fossils  in  them,  there  is 
often  considerable  difficulty  in  telling  whether  they  were  laid 
in  water  or  in  air.  In  either  case  they  soon  become  consolidated. 
Chemical  deposits  from  percolating  waters  fill  the  interstices, 
and  the  bed  of  loose  fragments  is  cemented  to  hard  rock. 

The  materials  of  which  tuffs  are  composed  are  easily  recog- 
nized as  volcanic  in  their  origin.  The  fragments  are  more  or 


256 


THE  ELEMENTS  OF  GEOLOGY 


less  cellular,  according  to  the  degree  to  which  they  were  dis- 
tended with  steam  when  in  a  molten  state,  and  even  in  the  finest 
dust  one  may  see  the  glass  or  the  crystals  of  lava  from  which  it 
was  derived.  Tuffs  often  contain  volcanic  bombs,  —  balls  of  lava 
which  took  shape  while  whirling  in  the  air,  and  solidified  before 
falling  to  the  ground. 

Ancient  volcanic  rocks.    It  is  in  these  materials  and  struc- 
tures which  we  have  described  that  volcanoes  leave  some  of 

their  most  enduring 
records.  Even  the  vol- 
canic rocks  of  the  earli- 
est geological  ages,  up- 
lifted after  long  burial 
beneath  the  sea  and  ex- 
posed to  view  by  deep 
erosion,  are  recognized 
and  their  history  read 
despite  the  many  changes 
which  they  may  have 
undergone.  A  sheet  of 
ancient  lava  may  be  distinguished  by  its  composition  from  the 
sediments  among  which  it  is  imbedded.  The  direction  of  its 
flow  lines  may  be  noted.  The  cellular  and  slaggy  surface  where 
the  pasty  lava  was  distended  by  escaping  steam  is  recognized 
by  the  amygdules  which  now  fill  the  ancient  steam  blebs.  In 
a  pile  of  successive  sheets  of  lava  each  flow  may  be  distinguished 
and  its  thickness  measured ;  for  the  surface  of  each  sheet  is 
glassy  and  scoriaceous,  while  beneath  its  upper  portions  the 
lava  of  each  flow  is  more  dense  and  stony.  The  length  of  time 
which  elapsed  before  a  sheet  was  buried  beneath  the  materials 
of  succeeding  eruptions  may  be  told  by  the  amount  of  weather- 
ing which  it  had  undergone,  the  depth  of  ancient  soil  —  now 
baked  to  solid  rock  —  upon  it,  and  the  erosion  which  it  had 
suffered  in  the  interval. 


FIG.  228.   Volcanic  Bombs,  Cinder  Cone, 
California 


VOLCANOES 


257 


If  the  flow  occurred  from  some  submarine  volcano,  we  may 
recognize  the  fact  by  the  sea-laid  sediments  which  cover  it,  fill- 
ing the  cracks  and  crevices  of  its  upper  surface  and  containing 
pieces  of  lava  washed  from  it  in  their  basal  layers. 

Long-buried  glassy  lavas  devitrify,  or  pass  to  a  stony  condi- 
tion, under  the  unceasing  action  of  underground  waters ;  but 
their  flow  lines  and  perlitic  and  spherulitic  structures  remain 
to  tell  of  their  original 
state. 

Ancient  tuffs  are 
known  by  the  frag- 
mental  character  of 
their  volcanic  material, 
even  though  they  hare 
been  altered  to  firm  rock. 
Some  remains  of  land 
animals  and  plants  may 
be  found  imbedded  to 
tell  that  the  beds  were  laid  in  open  air ;  while  the  remains  of 
marine  organisms  would  prove  as  surely  that  the  tuffs  were 
deposited  in  the  sea. 

In  these  ways  ancient  volcanoes  have  been  recognized  near 
Boston,  in  southeastern  Pennsylvania,  about  Lake  Superior,  and 
in  other  regions  of  the  United  States. 


FIG.  229.  A  Volcanic  Cone,  Arizona 


THE  LIFE  HISTORY  OF  A  VOLCANO 

The  invasion  of  a  region  by  volcanic  forces  is  attended  by 
movements  of  the  crust  heralded  by  earthquakes.  A  fissure  or 
a  pipe  is  opened  and  the  building  of  the  cone  or  the  spreading 
of  wide  lava  sheets  is  begun. 

Volcanic  cones.  The  shape  of  a  volcanic  cone  depends  chiefly 
on  the  materials  erupted.  Cones  made  of  fragments  may  have 
sides  as  steep  as  the  angle  of  repose,  which  in  the  case  of  coarse 


258 


THE  ELEMENTS  OF   GEOLOGY 


scoria  is  sometimes  as  high  as  thirty  or  forty  degrees.  About 
the  base  of  the  mountain  the  finer  materials  erupted  are  spread 
in  more  gentle  slopes,  and  are  also  washed  forward  by  rams  and 
streams.  The  normal  profile  is  thus  a  symmetric  cone  with  a 
flaring  base. 

Cones  built  of  lava  vary  in  form  according  to  the  liquidity 
of  the  lava.    Domes  of  gentle  slope,  as  those  of  Hawaii,  for 


FIG.  230.   Sarcoui,  a  Trachyte  Dome,  France 

example,  are  formed  of  basalt,  which  flows  to  long  distances 
before  it  congeals.  When  superheated  and  emitted  from  many 
vents,  this  easily  melted  lava  builds  great  plateaus,  such  as  that 
of  Iceland.  On  the  other  hand,  lavas  less  fusible,  or  poured  out 
at  a  lower  temperature,  stiffen  when  they  have  flowed  but  a 
short  distance,  and  accumulate  in  a  steep  cone.  Trachyte  has 
been  extruded  in  a  state  so  viscid  that  it  has  formed  steep- 
sided  domes  like  that  of  Sarcoui  (Fig.  230). 


VOLCANOES 


259 


Most  volcanoes  are  built,  like  Vesuvius,  both  of  lava  flows  and 
of  tuffs,  and  sections  show  that  the  structure  »of  the  cone  consists 
of  outward-dipping,  alternating  layers  of  lava,  scoria,  and  ashes. 


FIG.  231.   Section  of  Vesuvius 

V,  Vesuvius;  S,  Somma,  a  mountainous  rampart  half  encircling  Vesuvius, 
and  like  it  built  of  outward-dipping  sheets  of  tuff  and  lava ;  a,  crystalline 
rocks ;  6,  marine  strata;  c,  tuffs  containing  seashells.  Which  is  the  older 
mountain,  Vesuvius  or  Somma  ?  Of  what  is  Somma  a  remnant  ?  Draw 
a  diagram  showing  its  original  outline.  Suggest  what  processes  may 
have  brought" it  to  its  present  form.  What  record  do  you  find  of  the 
earliest  volcanic  activity  ?  What  do  you  infer  as  to  the  beginnings  of 
the  volcano  ? 

From  time  to  tune  the  cone  is  rent  by  the  violence  of  explo- 
sions and  by  the  weight  of  the  column  of  lava  in  the  pipe. 
The  fissures  are  filled  with  lava  and  some  discharge  on  the 
sides  of  the  mountain,  building  parasitic  cones,  while  all  form 
dikes,  which  strengthen 
the  pile  with  ribs  of 
hard  rock  and  make  it 
more  difficult  to  rend. 

Great  catastrophes 


Scale  of  Miles. 
FIG.  232.   Crater  Lake,  Oregon 


are  recorded  in  the  How  wide  and  how  deep  is  the  basin  which  holds 

the  lake  ?  The  mountain  walls  which  inclose 

Shape  of  some  volcanoes  it  are  made  of  outward-dipping  sheets  of  lava. 

Draw  a  diagram  restoring  the  volcano  of  which 
they  are  the  remnant.  No  volcanic  fragments 
of  the  same  nature  as  the  materials  of  which 
the  volcano  is  built  are  found  about  the  region. 
What  theory  of  the  destruction  of  the  cone  does 
this  fact  favor?  W,  Wizard  Island,  is  a  cinder 
cone.  When  was  it  built? 


which  consist  of  a  circu- 
lar run,  perhaps  miles 
in  diameter,  inclosing  a 
vast  crater  or  a  caldera 


within  which  small 
cones  may  rise.  We  may  infer  that  at  some  time  the  top  of 
the  mountain  has  been  blown  off,  or  has  collapsed  and  been 
engulfed  because  some  reservoir  beneath  had  been  emptied  by 
long-continued  eruptions  (Fig.  232). 


260 


THE  ELEMENTS  OF  GEOLOGY 


The  cone-building  stage  may  be  said  to  continue  until  erup- 
tions of  lava  and  fragmental  materials  cease  altogether.  Sooner 
or  later  the  volcanic  forces  shift  or  die  away,  and  no  further 
eruptions  add  to  the  pile  or  replace  its  losses  by  erosion  during 
periods  of  repose.  .Gases  however  are  still  emitted,  and,  as  sul- 
phur vapors  are  conspicuous  among  them,  such  vents  are  called 
solfataras.  Mount  Hood,  in  Oregon,  is  an  example  of  a  volcano 

sunk  to  this  stage.  From 
a  steaming  rift  on  its 
side  there  rise  sulphur- 
'ous  fumes  which,  half 
a  mile  down  the  wind, 
will  tarnish  a  silver  coin. 
Geysers  and  hot 
springs.  The  hot 
springs  of  volcanic  re- 
gions are  among  the 
last  vestiges  of  volcanic 
heat.  Periodically  erup- 
tive boiling  springs  are 
termed  geysers.  In  each 
of  the  geyser  regions  of 
the  earth  —  the  Yellow- 
stone  National  Park, 
Iceland,  and  New  Zea- 
land— the  ground  water 
of  the  locality  is  sup- 
posed to  be  heated  by  ancient  lavas  that,  because  of  the  poor 
conductivity  of  the  rock,  still  remain  hot  beneath  the  surface. 

Old  Faithful,  one  of  the  many  geysers  of  the  Yellowstone  National 
Park,  plays  a  fountain  of  boiling  water  a  hundred  feet  in  air;  while 
clouds  of  vapor  from  the  escaping  steam  ascend  to  several  times  that 
height.  The  eruptions  take  place  at  intervals  of  from  seventy  to  ninety 
minutes.  In  repose  the  geyser  is  a  quiet  pool,  occupying  a  craterlike 


FIG.  233.  Old  Faithful  Geyser  in  Eruption, 
Yellowstone  National  Park 


VOLCANOES 


261 


depression  in  a  conical  mound  some  twelve  feet  high.  The  conduit  of 
the  spring  is  too  irregular  to  be  sounded.  The  mound  is  composed  of 
porous  silica  deposited  by  the  waters  of  the  geyser. 

Geysers  erupt  at  intervals  instead  of  continuously  boiling, 
because  their  long,  narrow,  and  often  tortuous  conduits  do  not 
permit  a  free  circulation  of  the  water.  After  an  eruption  the 
tube  is  refilled  and  the  water  again  gradually  becomes  heated 


FIG.  234.   Terrace  and  Cones  of  Siliceous  Sinter  deposited  by  Geysers, 
Yellowstone  National  Park 

Deep  in  the  tube  where  it  is  in  contact  with  hot  lavas  the 
water  sooner  or  later  reaches  the  boiling  point,  and  bursting 
into  steam  shoots  the  water  above  it  high  in  air. 

Carbonated  springs.  After  all  the  other  signs  of  life  have 
gone,  the  ancient  volcano  may  emit  carbon  dioxide  as  its  dying 
breath.  The  springs  of  the  region  may  long  be  charged  with 
carbon  dioxide,  or  carbonated,  and  where  they  rise  through 
limestone  may  be  expected  to  deposit  large  quantities  of  traver- 
tine. We  should  remember,  however,  that  many  carbonated 
springs,  and  many  hot  springs,  are  wholly  independent  of 
volcanoes. 


262 


THE  ELEMENTS  OF  GEOLOGY 


The  destruction  of  the  cone.    As  soon  as  the  volcanic  cone 
ceases  to  grow  by  eruptions  the  agents  of  erosion  begin  to  wear 


FIG.  235.  Mount  Shasta,  California 

it  down,  and  the  length  of  time  that  has  elapsed  since  the  period 
of  active  growth  may  be  roughly  measured  by  the  degree  to 
which  the  cone  has  been  dissected.  We  infer  that  Mount  Shasta, 


FIG.  236.    Mount  Hood,  Oregon 

whose  conical  shape  is  still  preserved  despite  the  gullies  one 
thousand  feet  deep  which  trench  its  sides  (Fig.  235),  is  younger 
than  Mount  Hood,  which  erosive  agencies  have  carved  to  a 


VOLCANOES 


263 


pyramidal  form  (Fig.  236).    The  pile  of  materials  accumulated 
about  a  volcanic  vent,  no  matter  how  vast  in  bulk,  is  at  last 


Scale  of  Miles 
FIG.  237.  Crandall  Volcano 

swept  entirely  away.  The  cone  of  a  volcano,  active  or  extinct, 
is  not  old  as  the  earth  counts  tune ;  volcanoes  are  short-lived 
geological  phenomena. 

Crandall  Volcano.  This 
name  is  given  to  a  dis- 
sected ancient  volcano  in 
the  Yellowstone  National 
Park,  which  once,  it  is 
estimated,  reared  its  head 
thousands  of  feet  above  the 
surrounding  country  and 
greatly  exceeded  in  bulk 
either  Mount  Shasta  or 
Mount  Etna.  Not  a  line 
of  the  original  mountain 
remains;  all  has  been  swept 
away  by  erosion  except 
some  four  thousand  feet  of 
the  base  of  the  pile.  This 
basal  wreck  now  appears 
as  a  rugged  region  about 
thirty  miles  in  diameter, 
trenched  by  deep  valleys 
and  cut  into  sharp  peaks 
and  precipitous  ridges.  In 
the  center  of  the  area  is  found  the  nucleus  (A7,  Fig.  237),  —  a  mass  of 
coarsely  crystalline  rock  that  congealed  deep  in  the  old  volcanic  pipe. 
From  it  there  radiate  in  all  directions,  like  the  spokes  of  a  wheel,  long 
dikes  whose  rock  grows  rapidly  finer  of  grain  as  it  leaves  the  vicinity  of 


FIG.  238.   Fossil  Tree  Trunks,  Yellowstone 

National  Park 
To  the  left  is  seen  a  mass  of  volcanic  breccia 


264  THE  ELEMENTS  OF  GEOLOGY 

the  once  heated  core.  The  remainder  of  the  base  of  the  ancient  moun- 
tain is  made  of  rudely  bedded  tuffs  and  volcanic  breccia,  with  occasional 
flows  of  lava,  some  of  the  fragments  of  the  breccia  measuring  as  much 
as  twenty  feet  in  diameter.  On  the  sides  of  canyons  the  breccia  is 
carved  by  rain  erosion  to  fantastic  pinnacles.  At  different  levels  in  the 
midst  of  these  beds  of  tuff  and  lava  are  many  old  forest  grounds.  The 
stumps  and  trunks  of  the  trees,  now  turned  to  stone,  still  in  many  cases 
stand  upright  where  once  they  grew  on  the  slopes  of  the  mountain  as  it 
was  building  (Fig.  238).  The  great  size  and  age  of  some  of  these  trees 
indicate  the  lapse  of  time  between  the  eruption  whose  lavas  or  tuffs 
weathered  to  the  soil  on  which  they  grew  and  the  subsequent  eruption 
which  buried  them  beneath  showers  of  stones  and  ashes. 

Near  the  edge  of  the  area  lies  Death  Gulch,  in  which  carbon  dioxide 
is  given  off  in  such  quantities  that  in  quiet  weather  it  accumulates  in  a 
heavy  layer  along  the  ground  and  suffocates  the  animals  which  may 
enter  it. 


CHAPTEE  XII 
UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN 

It  is  because  long-continued  erosion  lays  bare  the  innermost 
anatomy  of  an  extinct  volcano,  and  even  sweeps  away  the 
entire  pile  with  much  of  the  underlying  strata,  thus  leaving 
the  very  roots  of  the  volcano  open  to  view,  that  we  are  able  to 
study  underground  volcanic  structures.  With  these  we  include, 
for  convenience,  intrusions  of  molten  rock  which  have  been 
driven  upward  into  the  crust,  but  which  may  not  have  suc- 
ceeded in  breaking  way  to  the  surface  and  establishing  a  vol- 
cano. All  these  structures  are  built  of  rock  forced  when  in  a 
fluid  or  pasty  state  into  some  cavity  which  it  has  found  or  made, 
and  we  may  classify  them  therefore,  according  to  the  shape  of 
the  molds  in  which  the  molten  rock  has  congealed,  as  (1)  dikes, 
(2)  volcanic  necks,  (3)  intrusive  sheets,  and  (4)  intrusive  masses. 

Dikes.  The  sheet  of  once  molten  rock  with  which  a  fissure 
has  been  filled  is  known  as  a  dike.  Dikes  are  formed  when 
volcanic  cones  are  rent  by  explosions  or  by  the  weight  of  the 
lava  column  hi  the  duct,  and  on  the  dissection  of  the  pile  they 
appear  as  radiating  vertical  ribs  cutting  across  the  layers  of 
lava  and  tuff  of  which  the  cone  is  built.  In  regions  under- 
going deformation  rocks  lying  deep  below  the  ground  are  often 
broken  and  the  fissures  are  filled  with  molten  rock  from  beneath, 
which  finds  no  outlet  to  the  surface.  Such  dikes  are  common 
in  areas  of  the  most  ancient  rocks,  which  have  been  brought  to 
light  by  long  erosion. 

In  exceptional  cases  dikes  may  reach  the  length  of  fifty  or 
one  hundred  miles.  They  vary  in  width  from  a  fraction  of  a 
foot  to  even  as  much  as  three  hundred  feet. 

265 


r 


206 


UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN     267 

Dikes  are  commonly  more  fine  of  grain  on  the  sides  than  in  the 
center,  and  may  have  a  glassy  and  crackled  surface  where  they  meet 
the  inclosing  rock.  Can  you  account  for  this  on  any  principle  which 
you  have  learned  ?  • 

Volcanic  necks.  The  pipe  of  a  volcano  rises  from  far  below 
the  base  of  the  cone,  —  from  the  deep  reservoir  from  which  its 


•N\ 


Fi<;.  240.   A  Dissected  Volcanic  Cone 

N,  volcanic  neck ;  I,  I,  lava-topped  table  mountains ;  t,  t,  beds  of  tuff ;  d,  d, 
dikes ;  dotted  lines  indicate  the  initial  profile 

eruptions  are  supplied.  When  the  volcano  has  become  extinct 
this  great  tube  remains  filled  with  hardened  lava.  It  forms  a 
cylindrical  core  of  solid  rock,  except  for  some  distance  below 
the  ancient  crater,  where  it  may  contain  a  mass  of  fragments 
which  had  fallen  back  into  the  chimney  after  being  hurled  into 
the  air. 

As  the  mountain  is  worn  down,  this  central  column  known 
as  the  volcanic  neck  is  left  standing  as  a  conical  hill  (Fig.  240). 
Even  when  every  other 
trace  of  the  volcano  has 
been  swept  away,  ero- 
sion will  not  have  passed 
below  this  great  stalk  on 
which  the  volcano  was 
borne  as  a  fiery  flower 
whose  site  it  remains 

to    mark.     In   volcanic      FlG'  241'  Mount  Johnson>  a  Volcanic  Neck 

near  Montreal 
regions  of  deep  denuda- 
tion volcanic  necks  rise  solitary  and  abrupt  from  the  surround- 
ing country  as  dome-shaped  hills.    They  are  marked  features  in 


268 


THE  ELEMENTS  OF  GEOLOGY 


the  landscape  in  parts  of  Scotland  and  in  the  St.  Lawrence  val- 
ley about  Montreal  (Fig.  241). 

Intrusive  sheets.  Sheets  of  igneous  rocks  are  sometimes 
found  interleaved  with  sedimentary  strata,  especially  in  regions 
where  the  rocks  have  been  deformed  and  have  suffered  from 
volcanic  action.  In  some  instances  such  a  sheet  is  seen  to  be 
contemporaneous  (p.  248).  In  other  instances  the  sheet  must 


FIG.  242.  The  Palisades  of  the  Hudson,  New  Jersey 

be  intrusive.  The  overlying  stratum,  as  well  as  that  beneath, 
has  been  affected  by  the  heat  of  the  once  molten  rock.  We 
infer  that  the  igneous  rock  when  in  a  molten  state  was  forced 
between  the  strata,  much  as  a  card  may  be  pushed  between  the 
leaves  of  a  closed  book.  The  liquid  wedged  its  way  between 
the  layers,  lifting  those  above  to  make  room  for  itself.  The 
source  of  the  intrusive  sheet  may  often  be  traced  to  some 
dike  (known  therefore  as  the  feeding  dike),  or  to  some  mass  of 
igneous  rock. 


UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN  269 


Intrusive  sheets  may  extend  a  score  and  more  of  miles,  and, 
like  the  longest  surface  flows,  the  most  extensive  sheets  consist 
of  the  more  fusible  and  fluid  lavas,  —  those  of  the  basic  class  of 
which  basalt  is  an  example.  Intrusive  sheets  are  usually  harder 
than  the  strata  in  which  they  lie  and  are  therefore  often  left  in 
relief  after  long  denudation  of  the  region  (Fig.  315). 

On  the  west  bank  of  the  Hudson  there  extends  from  New  York  Bay 
north  for  thirty  miles  a  bold  cliff  several  hundred  feet  high,  —  the 
Palisades  of  the  Hudson.  It  is 
the  outcropping  edge  of  a  sheet 
of  ancient  igneous  rock,  which 


FIG.  243.   Diagram  of  the  Palisades  of 
the  Hudson 

i,  intrusive  sheet;  s,  sandstone;  d,  feeding 
dike;  HR,  Hudson  River 


rests   on   stratified   sandstones 

and  is  overlain  by  strata  of  the 

same    series.     Sandstones   and 

lava  sheet  together  dip  gently 

to  the  west  and  the  latter  disappears  from  view  two  miles  back  from 

the  river. 

It  is  an  interesting  question  whether  the  Palisades  sheet  is  contem- 
poraneous or  intrusive.  Was  it  outpoured  on  the  sandstones  beneath  it 
when  they  formed  the  floor  of  the  sea,  and  covered  forthwith  by  the 
sediments  of  the  strata  above,  or  was  it  intruded  among  these  beds  at  a 

later  date  ? 

The  latter  is  the 
case  ;  for  the  overly- 
ing stratum  is  in- 

Scale  of  Miles  tensely   baked    along 

FIG.  244.  Section  of  Electric  Peak,  E,  and  Gray    the   zone   of  contact. 
Peak,  G,  Yellowstone  National  Park  ^  the  west  edge  of 

Intrusive  sheets  and  masses  of  igneous  rock  are  drawn     the  sheet  is  found  the 
in  black  dike  in  which  the  lava 

rose  to  force  its  way  far  and  wide  between  the  strata. 

Electric  Peak,  one  of  the  prominent  mountains  of  the  Yellowstone 
National  Park,  is  carved  out  of  a  mass  of  strata  into  which  many 
sheets  of  molten  rock  have  been  intruded.  The  western  summit  con- 
sists of  such  a  sheet  several  hundred  feet  thick.  Studying  the  section 
of  Figure  244,  what  inference  do  you  draw  as  to  the  source  of  these 
intrusive  sheets? 


270 


THE   ELEMENTS   OF   GEOLOGY 


INTRUSIVE  MASSES 

Bosses.    This  iiame   is  generally   applied  to  huge  irregular 
masses  of  coarsely  crystalline  igneous  rock  lying  in  the  midst 

of  other  formations.  Bosses 
vary  greatly  in  size  and  may 
reach  scores  of  miles  in  ex- 
tent. Seldom  are  there  any 
evidences  found  that  bosses 
ever  had  connection  with 
the  surface.  On  the  other 
hand,  it  is  often  proved  that 
they  have  been  driven,  or 
FIG.  245.  Stone  Mountain,  Georgia,  a  have  melted  their  way,  up- 
ward into  the  formations  in 

which  they  lie ;  for  they  give  off  dikes  and  intrusive  sheets,  and 
have  profoundly  altered  the  rocks  about  them  by  their  heat. 

The  texture  of  the  rock 
of  bosses  proves  that  con- 
solidation proceeded  slowly 
and  at  great  depths,  and  it 
is  only  because  of  vast  de- 
nudation that  they  are  now 
exposed  to  view.  Bosses  are 
commonly  harder  than  the 
rocks  about  them,  and  stand 
up,  therefore,  as  rounded 
hills  and  mountainous 
ridges  long  after  the  sur- 
rounding country  has  worn 
to  a  low  plain  (Fig.  245). 
The  base  of  bosses  is  in- 


definite   or    undetermined,    v 

FIG.  246.   Map   of    Granite  Bosses  near 

and    in    this    respect    they          Baltimore  (areas  horizontally  lined) 


UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN      271 


differ  from  laccoliths.  Some  bosses  have  broken  and  faulted 
the  overlying  beds;  some  have  forced  the  rocks  aside  and 
melted  them  away. 

The  Spanish  Peaks  of  southeastern  Colorado  were  formed  by  the 
upthrust  of  immense  masses  of  igneous  rock,  bulging  and  breaking  the 
overlying  strata.  On  one  side  of  the  mountains  the  throw  of  the  fault 
is  nearly  a  mile,  and  fragments  of  deep-lying  beds  were  dragged  upward 
by  the  rising  masses.  The  adjacent  rocks  were  altered  by  heat  to  a 
distance  of  several  thousand  feet.  No  evidence  appears  that  the  molten 
rock  ever  reached  the  surface,  and  if  volcanic  eruptions  ever  took  place 
either  in  lava  flows  or  fragmental  materials,  all  traces  of  them  have 
been  effaced.  The  rock  of  the  intrusive  masses  is  coarsely  crystalline, 
and  no  doubt  solidified  slowly  under  the  pressure  of  vast  thicknesses  of 
overlying  rock,  now  mostly  removed  by  erosion. 

A  magnificent  system  of  dikes  radiates  from  the  Peaks  to  a  distance 
of  fifteen  miles,  some  now  being  left  by  long  erosion  as  walls  a  hundred 
feet  in  height  (Fig.  239).  Intrusive  sheets  fed  by  the  dikes  penetrate 
the  surrounding  strata,  and  their  edges  are  cut  by  canyons  as  much  as 
twenty-five  miles  from  the  mountain.  In  these  strata  are  valuable  beds 
of  lignite,  an  imperfect  coal,  which  the  heat  of  dikes  and  sheets  has 
changed  to  coke. 

Laccoliths.  The  laccolith  (Greek  laccos,  cistern;  lithos,  stone) 
is  a  variety  of  intrusive  masses  in  which  molten  rock  has 
spread  between  the  strata, 
and,  lifting  the  strata  above 
it  to  a  dome-shaped  form, 
has  collected  beneath  them 
in  a  lens-shaped  body  with 
a  flat  base. 

The     Henry    Mountains,     a 

,   ,  ,     ,     ,         ,  FIG.  247.   Section  of  a  Laccolith 

small  group  of  detached  peaks 

in  southern  Utah,  rise  from  a  plateau  of  horizontal  rocks.  Some  of  the 
peaks  are  carved  wholly  in  separate  domelike  uplifts  of  the  strata  of 
the  plateau.  In  others,  as  Mount  Killers,  the  largest  of  the  group,  there 
is  exposed  on  the  summit  a  core  of  igneous  rock  from  which  the  sedi- 
mentary rocks  of  the  flanks  dip  steeply  outward  in  all  directions.  In 


272         THE  ELEMENTS  OF  GEOLOGY 

still  others  erosion  has  stripped  oft'  the  covering  strata  and  has  laid  bare 
the  core  to  its  base ;  and  its  shape  is  here  seen  to  be  that  of  a  plano- 
convex lens  or  a  baker's  bun,  its  flat  base  resting  on  the  undisturbed 
bedded  rocks  beneath.  The  structure  of  Mount  Killers  is  shown  in 
Figure  248.  The  nucleus  of  igneous  rock  is  four  miles  in  diameter 
and  more  than  a  mile  in  depth. 

Regional  intrusions.  These  vast  bodies  of  igneous  rock,  which 
may  reach  hundreds  of  miles  in  diameter,  differ  little  from  bosses 
except  in  their  immense  bulk.  Like  bosses,  regional  intrusions 

give  off  dikes  and  sheets 
and  greatly  change  the 
rocks  about  them  by 
their  heat.  They  are 
now  exposed  to  view 
only  because  of  the  pro- 
FIG.  248.  Section  of  Mount  Killers  found  denudation  which 

has  removed  the  upheaved  dome  of  rocks  beneath  which  they 
slowly  cooled.  Such  intrusions  are  accompanied  —  whether  as 
cause  or  as  effect  is  still  hardly  known  —  by  deformations,  and 
their  masses  of  igneous  rock  are  thus  found  as  the  core  of  many 
great  mountain  ranges.  The  granitic  masses  of  which  the  Bitter 
Root  Mountains  and  the  Sierra  Nevadas  have  been  largely  carved 
are  each  more  than  three  hundred  miles  in  length.  Immense 
regional  intrusions,  the  cores  of  once  lofty  mountain  ranges,  are 
found  upon  the  Laurentian  peneplain. 

Physiographic  effects  of  intrusive  masses.  We  have  already 
seen  examples  of  the  topographic  effects  of  intrusive  masses  in 
Mount  Killers,  the  Spanish  Peaks,  and  in  the  great  mountain 
ranges  mentioned  in  the  paragraph  on  regional  intrusions, 
although  in  the  latter  instances  these  effects  are  entangled 
with  the  effects  of  other  processes.  Masses  of  igneous  rock 
cannot  be  intruded  within  the  crust  without  an  accompanying 
deformation  on  a  scale  corresponding  to  the  bulk  of  the  in- 
truded mass.  The  overlying  strata  are  arched  into  hills  or 


UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN  273 

mountains,  or,  if  the  molten  material  is  of  great  extent,  the  strata 
may  Conceivably  be  floated  upward  to  the  height  of  a  plateau. 
We  may  suppose  that  the  transference  of  molten  matter  from  one 
region  to  another  may  be  among  the  causes  of  slow  subsidences 
and  elevations.  Intrusions  give  rise  to  fissures,  dikes,  and  in- 
trusive sheets,  and  these  dislocations  cannot  fail  to  produce  earth- 
quakes. Where  intrusive  masses  open  communication  with  the 
surface,  volcanoes  are  established  or  fissure  eruptions  occur  such 
as  those  of  Iceland. 

THE  INTRUSIVE  EOCKS 

The  igneous  rocks  are  divided  into  two  general  classes, — the 
volcanic  or  eruptive  rocks,  which  have  been  outpoured  in  open 
air  or  on  the  floor  of  the  sea,  and  the  intrusive  rocks,  which 
have  been  intruded  within  the  rocks  of  the  crust  and  have  solid- 
ified below  the  surface.  The  two  classes  are  alike  in  chemical 
composition  and  may  be  divided  into  acidic  and  basic  groups. 
In  texture  the  intrusive  rocks  differ  from  the  volcanic  rocks 
because  of  the  different  conditions  under  which  they  have 
solidified.  They  cooled  far  more  slowly  beneath  the  cover  of 
the  rocks  into  which  they  were  pressed  than  is  permitted  to  lava 
flows  in  open  air.  Their  constituent  minerals  had  ample  oppor- 
tunity to  sort  themselves  and  crystallize  from  the  fluid  mixture, 
and  none  of  that  mixture  was  left  to  congeal  as  a  glassy  paste. 

They  consolidated  also  under  pressure.  They  are  never  sco- 
riaceous,  for  the  steam  with  which  they  were  charged  was  not 
allowed  to  expand  and  distend  them  with  steam  blebs.  In  the 
rocks  of  the  larger  intrusive  masses  one  may  see  with  a  power- 
ful microscope  exceedingly  minute  cavities,  to  be  counted  by 
many  millions  to  the  cubic  inch,  in  which  the  gaseous  water 
which  the  mass  contained  was  held  imprisoned  under  the  im- 
mense pressure  of  the  overlying  rocks. 

Naturally  these  characteristics  are  best  developed  in  the 
intrusives  which  cooled  most  slowly,  i.e.  in  the  deepest-seated 


274  THE  ELEMENTS  OF  GEOLOGY 

and  largest  masses ;  while  in  those  which  cooled  more  rapidly, 
as  in  dikes  and  sheets,  we  find  gradations  approaching  the 
texture  of  surface  flows. 

Varieties  of  the  intrusive  rocks.  We  will  now  describe  a 
few  of  the  varieties  of  rocks  of  deep-seated  intrusions.  All  are 
even  grained,  consisting  of  a  mass  of  crystalline  grains  formed 
during  one  continuous  stage  of  solidification,  and  no  porphyritic 
crystals  appear  as  in  lavas. 

Granite,  as  we  have  learned  already,  is  composed  of  three 
minerals,  —  quartz,  feldspar,  and  mica.  According  to  the  color  of 
the  feldspar  the  rock  may  be  red,  or  pink,  or  gray.  Hornblende 
—  a  black  or  dark  green  mineral,  an  iron-magnesian  silicate, 
about  as  hard  as  feldspar  —  is  sometimes  found  as  a  fourth 
constituent,  and  the  rock  is  then  known  as  hornblendic  granite, 
Granite  is  an  acidic  rock  corresponding  to  rhyolite  in  chemical 
composition.  We  may  believe  that  the  same  molten  mass  which 
supplies  this  acidic  lava  in  surface  flows  solidifies  as  granite 
deep  below  ground  in  the  volcanic  reservoir. 

Syenite,  composed  of  feldspar  and  mica,  has  consolidated 
from  a  less  siliceous  mixture  than  has  granite. 

Diorite,  still  less  siliceous,  is  composed  of  hornblende  and 
feldspar,—1-  the  latter  mineral  being  of  different  variety  from  the 
feldspar  of  granite  and  syenite. 

Galibro,  a  typical  basic  rock,  corresponds  to  basalt  in  chemical 
composition.  It  is  a  dark,  heavy,  coarsely  crystalline  aggregate 
of  feldspar  and  auyite  (a  dark  mineral  allied  to  hornblende).  It 
often  contains  magnetite  (the  magnetic  black  oxide  of  iron)  and 
olivine  (a  greenish  magnesian  silicate). 

In  the  northern  states  all  these  types,  and  many  others  also 
of  the  vast  number  of  varieties  of  intrusive  rocks,  can  be  found 
among  the  rocks  of  the  drift  brought  from  the  areas  of  igneous 
rock  in  Canada  and  the  states  of  our  northern  border. 

Summary.  The  records  of  geology  prove  that  since  the  earli- 
est of  their  annals  tremendous  forces  have  been  active  iii  the 


UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN     275 

earth.    In  all  the   past,  under  pressures  inconceivably  great, 
molten  rock  has  been  driven  upward  into  the  rocks  of  the  crust. 


FIG.  249.  Ground  Plan  of  Dikes 
in  Granite.  (Scale  80  feet  to 
the  inch) 

What  is  the  relative  age  of  the  dikes 
aa,  bb,  and  cc  ? 


FIG.  250,  A  and  B.  Mountains 
of  coarsely  Crystalline  Ig- 
neous Rock  i,  surrounded 
by  Sedimentary  Strata  s 
ands' 

Copy  each  diagram  and  complete 
it,  so  as  to  show  whether  the 
mass  of  igneous  rock  is  a 
volcanic  neck,  a  boss,  or  a 
laccolith 


It  has  squeezed  into  fissures  forming  dikes;  it  has  burrowed 
among  the  strata  as  intrusive  sheets ;  it  has  melted  the  rocks 
away  or  lifted  the  overlying  strata,  filling  the  chambers  which 
it  has  made  with  intrusive  masses.  During 
all  geological  ages  molten  rock  has  found 
way  to  the  surface,  and  volcanoes  have 
darkened  the  sky  with  clouds  of  ashes  and 
poured  streams  of  glowing  lava  down  their 


FIG.  251. 

1,  limestone;  2,  tuff; 
3,  5,  7,  shale  with 
marine  shells;  4,  6, 
lava,  dotted  portions 
scoriaceous.  Give  the 
history  recorded  in 
this  section 


FIG.  252. 

a,  sedimentary  strata  with  intrusive  sheets;  b,  sedi- 
mentary strata;  c,  lava  flow;  d,  dike.  Give  the 
succession  of  events  recorded  in  this  section 


276 


THE  ELEMENTS  OF  GEOLOGY 


sides.  The  older  strata,  —  the  strata  which  have  been  most 
deeply  buried,  —  and  especially  those  which  have  suffered  most 
from  folding  and  from  fracture,  show  the  largest  amount  of  igne- 
ous intrusions.  The  molten  rock  which  has  been 
driven  from  the  earth's  interior  to  within  the 
crust  or  to  the  surface  during  geologic  time  must 
be  reckoned  in  millions  of  cubic  miles. 

IB 


FIG.  253 

Which  of  the  lava  sheets  of 
this  section  are  contem- 
poraneous and  which  in- 
trusive,—  A,  whose  upper 
surface  is  overlain  with 
a  conglomerate  of  rolled 
lava  pebbles ;  B,  the  cracks 
and  seams  of  whose  upper 
surface  are  filled  with  the 
material  of  the  overly- 
ing sandstone  ;  C,  which 
breaks  across  the  strata  in 
which  it  is  imbedded ;  D, 
which  includes  fragments 
of  both  the  underlying 
and  overlying  strata  and 
penetrates  their  crevices 
and  seams? 


FIG.  2;">4.  Mato  Tepee,  Wyoming 
This  magnificent  tower  of  igneous  rock  three 
hundred  feet  in  height  has  been  called  by 
some  a  volcanic  neck.  Is  the  direction  of  the 
columns  that  which  would  obtain  in  the 
cylindrical  pipe  of  a  volcano?  The  tower  is 
probably  the  remnant  of  a  small  laccolith,  an 
outlying  member  of  a  group  of  laccoliths 
situated  not  far  distant 


THE  INTERIOR  CONDITION  OF  THE  EARTH  AND  CAUSES 
OF  VULCANISM  AND  DEFORMATION 

The  problems  of  volcanoes  and  of  deformation  are  so  closely 
connected  witli  that  of  the  earth's  interior  that  we  may  consider 
them  together.  Few  of  these  problems  are  solved,  and  we  may 
only  state  some  known  facts  and  the  probable  conclusions 
which  may  be  drawn  as  inferences  from  them. 


UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN     277 

The  interior  of  the  earth  is  hot.  Volcanoes  prove  that  in 
many  parts  of  the  earth  there  exist  within  reach  of  the  sur- 
face regions  of  such  intense  heat  that  the  rock  is  in  a  molten 
condition.  Deep  wells  and  mines  show  everywhere  an  increase 
in  temperature  below  the  surface  shell  affected  by  the  heat  of 
summer  and  the  cold  of  winter,  —  a  shell  in  temperate  latitudes 
sixty  or  seventy  feet  thick.  Thus  in  a  boring  more  than  a  mile 
deep  at  Schladebach,  Germany,  the  earth  grows  warmer  at  the 
rate  of  1°  F.  for  every  sixty-seven  feet  as  we  descend.  Taking 
the  average  rate  of  increase  at  one  degree  for  every  sixty  feet 
of  descent,  and  assuming  that  this  rate,  observed  at  the  moderate 
distances  open  to  observation,  continues  to  at  least  thirty-five 
miles,  the  temperature  at  that  depth  must  be  more  than  three 
thousand  degrees,  —  a  temperature  at  which  all  ordinary  rocks 
would  melt  at  the  earth's  surface.  The  rate  of  increase  in  tem- 
perature probably  lessens  as  we  go  downward,  and  it  may  not  be 
appreciable  below  a  few  hundred  miles.  But  there  is  no  reason 
to  doubt  that  the  interior  of  the  earth  is  intensely  hot.  Below 
a  depth  of  one  or  two  score  miles  we  may  imagine  the  rocks 
everywhere  glowing  with  heat. 

Although  the  heat  of  the  ulterior  is  great  enough  to  melt  all 
rocks  at  atmospheric  pressure,  it  does  not  follow  that  the  interior 
is  fluid.  Pressure  raises  the  fusing  point  of  rocks,  and  the 
weight  of  the  crust  may  keep  the  interior  in  what  may  be 
called  a  solid  state,  although  so  hot  as  to  be  a  liquid  or  a  gas 
were  the  pressure  to  be  removed. 

The  interior  of  the  earth  is  rigid  and  heavy.  The  earth 
behaves  as  a  globe  more  rigid  than  glass  under  the  attractions 
of  the  sun  and  moon.  It  is  not  deformed  by  these  stresses  as 
is  the  ocean  hi  the  tides,  proving  that  it  is  not  a  fluid  ball  cov- 
ered with  a  yielding  crust  a  few  miles  thick.  Earthquakes  pass 
through  the  earth  faster  than  they  would  were  it  of  solid  steel. 
Hence  the  rocks  of  the  interior  are  highly  elastic,  being  brought 
by  pressure  to  a  compact,  continuous  condition  unbroken  by 


278  THE  ELEMENTS  OF  GEOLOGY 

the  cracks  and  vesicles  of  surface  rocks.  The  interior  of  the 
earth  is  rigid. 

The  common  rocks  of  the  crust  are  about  two  and  a  half 
times  heavier  than  water,  while  the  earth  as  a  whole  weighs 
five  and  six-tenths  times  as  much  as  a  globe  of  water  of  the  same 
size.  The  interior  is  therefore  much  more  heavy  than  the  crust. 
This  may  be  caused  in  part  by  compression  of  the  interior 
under  the  enormous  weight  of  the  crust,  and  in  part  also  by 
an  assortment  of  material,  the  heavier  substances,  such  as  the 
heavy  metals,  having  gravitated  towards  the  center. 

Between  the  crust,  which  is  solid  because  it  is  cool,  and  the 
interior,  which  is  hot  enough  to  melt  were  it  not  for  the  pressure 
which  keeps  it  dense  and  rigid,  there  may  be  an  intermediate 
zone  in  which  heat  and  pressure  are  so  evenly  balanced  that 
here  rock  liquefies  whenever  and  wherever  the  pressure  upon 
it  may  be  relieved  by  movements  of  the  crust.  It  is  perhaps 
from  such  a  subcrustal  layer  that  the  lava  of  volcanoes  is 
supplied. 

The  causes  of  volcanic  action.  It  is  now  generally  believed 
that  the  heat  of  volcanoes  is  that  of  the  earth's  interior.  Other 
causes,  such  as  friction  and  crushing  in  the  making  of  moun- 
tains and  the  chemical  reactions  between  oxidizing  agents  of 
the  crust  and  the  unoxidized  interior,  have  been  suggested,  but 
to  most  geologists  they  seem  inadequate. 

There  is  much  difference  of  opinion  as  to  the  force  which 
causes  molten  rock  to  rise  to  the  surface  in  the  ducts  of  vol- 
canoes. Steam  is  so  evidently  concerned  in  explosive  eruptions 
that  many  believe  that  lava  is  driven  upward  by  the  expansive 
force  of  the  steam  with  which  it  is  charged,  much  as  a  viscid 
liquid  rises  and  boils  over  in  a  test  tube  or  kettle. 

But  in  quiet  eruptions,  and  still  more  in  the  irruption  of  intru- 
sive sheets  and  masses,  there  is  little  if  any  evidence  that  steam 
is  the  driving  force.  It  is  therefore  believed  by  many  geologists 
that  it  is  pressure  due  to  crustal  movements  and  internal  stresses 


UNDERGROUND  STRUCTURES  OF  IGNEOUS  ORIGIN  279 

which  squeezes  molten  rock  from  below  into  fissures  and  ducts 
in  the  crust.  It  is  held  by  some  that  where  considerable  water 
is  supplied  to  the  rising  column  of  lava,  as  from  the  ground 
water  of  the  surrounding  region,  and  where  the  lava  is  viscid 
so  that  steam  does  not  readily  escape,  the  eruption  is  of  the 
explosive  type ;  when  these  conditions  do  not  obtain,  the  lava 
outwells  quietly,  as  in  the  Hawaiian  volcanoes.  It  is  held  by 
others  not  only  that  volcanoes  are  due  to  the  outflow  of  the 
earth's  deep-seated  heat,  but  also  that  the  steam  and  other 
emitted  gases  are  for  the  most  part  native  to  the  earth's  in- 
terior and  never  have  had  place  in  the  circulation  of  atmos- 
pheric and  ground  waters. 

Volcanic  action  and  deformation.  Volcanoes  do  not  occur  on 
wide  plains  or  among  ancient  mountains.  On  the  other  hand, 
where  movements  of  the  earth's  crust  are  in  progress  in  the 
uplift  of  high  plateaus,  and  still  more  in  mountain  making, 
molten  rock  may  reach  the  surface,  or  may  be  driven  upward 
toward  it  forming  great  intrusive  masses.  Thus  extensive  lava 
flows  accompanied  the  upheaval  of  the  block  mountains  of  west- 
ern North  America  and  the  uplift  of  the  Colorado  plateau.  A 
line  of  recent  volcanoes  may  be  traced  along  the  system  of  rift 
valleys  which  extends  from  the  Jordan  and  Dead  Sea  througli 
eastern  Africa  to  Lake  Nyassa.  The  volcanoes  of  the  Andes 
show  how  conspicuous  volcanic  action  may  be  in  young  rising 
ranges.  Folded  mountains  often  show  a  core  of  igneous  rock, 
which  by  long  erosion  has  come  to  form  the  axis  and  the  highest 
peaks  of  the  range,  as  if  the  molten  rock  had  been  squeezed  up 
under  the  rising  upfolds.  As  we  decipher  the  records  of  the 
rocks  in  historical  geology  we  shall  see  more  fully  how,  in  all 
the  past,  volcanic  action  has  characterized  the  periods  of  great 
crustal  movements,  and  how  it  has  been  absent  when  and  where 
the  earth's  crust  has  remained  comparatively  at  rest. 

The  causes  of  deformation.  As  the  earth's  interior,  or  nucleus, 
is  highly  heated  it  must  be  constantly  though  slowly  losing  its 


280         THE  ELEMENTS  OF  GEOLOGY 

heat  by  conduction  through  the  crust  and  into  space ;  and  since 
the  nucleus  is  cooling  it  must  also  be  contracting.  The  nucleus 
has  contracted  also  because  of  the  extrusion  of  molten  matter, 
the  loss  of  constituent  gases  given  off  in  volcanic  eruptions,  and 
(still  more  important)  the  compression  and  consolidation  of  its 
material  under  gravity.  As  the  nucleus  contracts,  it  tends  to 
draw  away  from  the  cooled  and  solid  crust,  and  the  latter  set- 
tles, adapting  itself  to  the  shrinking  nucleus  much  as  the  skin 
of  a  withering  apple  wrinkles  down  upon  the  shrunken  fruit. 
The  unsupported  weight  of  the  spherical  crust  develops  enor- 
mous tangential  pressures,  similar  to  the  stresses  of  an  arch 
or  dome,  and  when  these  lateral  thrusts  accumulate  beyond 
the  power  of  resistance  the  solid  rock  is  warped  and  folded  and 
broken. 

Since  the  planet  attained  its  present  mass  it  has  thus  been 
lessening  in  volume.  Notwithstanding  local  and  relative  up- 
heavals the  earth's  surface  on  the  whole  has  drawn  nearer  and 
nearer  to  the  center.  The  portions  of  the  lithosphere  which 
have  been  carried  down  the  farthest  have  received  the  waters 
of  the  oceans,  while  those  portions  which  have  been  carried 
down  the  least  have  emerged  as  continents. 

Although  it  serves  our  convenience  to  refer  the  movements 
of  the  crust  to  the  sea  level  as  datum  plane,  it  is  understood 
that  this  level  is  by  no  means  fixed.  Changes  in  the  ocean 
basins  increase  or  reduce  their  capacity  and  thus  lower  or  raise 
the  level  of  the  sea.  But  since  these  basins  are  connected,  the 
effect  of  any  change  upon  the  water  level  is  so  distributed  that 
it  is  far  less  noticeable  than  a  corresponding  change  would  be 
upon  the  land. 


CHAPTER   XIII 
METAMORPHISM    AND    MINERAL   VEINS 

Under  the  action  of  internal  agencies  rocks  of  all  kinds  may 
be  rendered  harder,  more  firmly  cemented,  and  more  crystalline. 
These  processes  are  known  as  metamorphism ,  and  the  rocks 
affected,  whether  originally  sedimentary  or  igneous,  are  called 
metamorphic  rocks.  We  may  contrast  with  metamorphism  the 
action  of  external  agencies  in  weathering,  which  render  rocks 
less  coherent  by  dissolving  their  soluble  parts  and  breaking 
down  their  crystalline  grams. 

Contact  metamorphism.  Rocks  beneath  a  lava  flow  or  in 
contact  with  igneous  intrusions  are  found  to  be  metamorphosed 
to  various  degrees  by  the  heat  of  the  cooling  mass.  The  adja- 
cent strata  may  be  changed  only  in  color,  hardness,  and  texture. 
Thus,  next  to  a  dike,  bituminous  coal  may  be  baked  to  coke  or 
anthracite,  and  chalk  and  limestone  to  crystalline  marble.  Sand- 
stone may  be  converted  into  quartzite,  and  shale  into  argillite, 
a  compact,  massive  clay  rock.  New  minerals  may  also  be  de- 
veloped. In  sedimentary  rocks  there  may  be  produced  crystals 
of  mica  and  of  garnet  (a  mineral  as  hard  as  quartz,  commonly 
occurring  in  red,  twelve-sided  crystals).  Where  the  changes  are 
most  profound,  rocks  may  be  wholly  made  over  in  structure  and 
mineral  composition. 

In  contact  metamorphism  thin  sheets  of  molten  rock  pro- 
duce less  effect  than  thicker  ones.  The  strongest  heat  effects 
are  naturally  caused  by  bosses  and  regional  intrusions,  and  the 
zone  of  change  about  them  may  be  several  miles  in  width.  In 
these  changes  heated  waters  and  vapors  from  the  masses  of 
igneous  rocks  undoubtedly  play  a  very  important  part. 

281 


282         THE  ELEMENTS  OF  GEOLOGY 

Which  will  be  more  strongly  altered,  the  rocks  about  a  closed  dike 
in  which  lava  began  to  cool  as  soon  as  it  filled  the  fissure,  or  the  rocks 
about  a  dike  which  opened  on  the  surface  and  through  which  the 
molten  rock  flowed  for  some  time? 

Taking  into  consideration  the  part  played  by  heated  waters,  which  will 
produce  the  most  far-reaching  metamorphism,  dikes  which  cut  across  the 
bedding  planes  or  intrusive  sheets  which  are  thrust  between  the  strata? 

Regional  metamorphism.  Metamorphic  rocks  occur  wide- 
spread in  many  regions,  often  hundreds  of  square  miles  in  area, 
where  such  extensive  changes  cannot  be  accounted  for  by 
igneous  intrusions.  Such  are  the  dissected  cores  of  lofty  moun- 
tains, as  the  Alps,  and  the  worn-down  bases  of  ancient  ranges, 
as  in  New  England,  large  areas  in  the  Piedmont  Belt,  and  the 
Laurentian  peneplain. 

.  In  these  regions  the  rocks  have  yielded  to  immense  pressure. 
They  have  been  folded,  crumpled,  and  mashed,  and  even  their 
minute  grains,  as  one  may  see  with  a  microscope,  have  often 
been  puckered,  broken,  and  crushed  to  powder.  It  is  to  these 
mechanical  movements  and  strains  which  the  rocks  have  suf- 
fered in  every  part  that  we  may  attribute  their  metamorphism, 
and  the  degree  to  which  they  have  been  changed  is  in  direct  pro- 
portion to  the  degree  to  which  they  have  been  deformed  and 
mashed. 

Other  factors,  however,  have  played  important  parts.  Rock 
crushing  develops  heat,  and  allows  a  freer  circulation  of  heated 
waters  and  vapors.  Thus  chemical  reactions  are  greatly  quick- 
ened ;  minerals  are  dissolved  and  redeposited  in  new  positions, 
or  their  chemical  constituents  may  recombine  in  new  minerals, 
entirely  changing  the  nature  of  the  rock,  as  when,  for  example, 
feldspar  recrystallizes  as  quartz  and  mica. 

Early  stages  of  metamorphism  are  seen  in  slate.  Pressure  has 
hardened  the  marine  muds,  the  arkose  (p.  186),  or  the  volcanic  ash 
from  which  slates  are  derived,  and  has  caused  them  to  cleave  by  the 
rearrangement  of  their  particles. 


METAMORPHISM  AND  MINERAL  VEINS  283 

Under  somewhat  greater  pressure,  slate  becomes  phyllite,  a  clay  slate 
whose  cleavage  surfaces  are  lustrous  with  flat-lying  mica  flakes.  The 
same  pressure  which  has  caused  the  rock  to  cleave  has  set  free  some  of 
its  mineral  constituents  along  the  cleavage  planes  to  crystallize  there 
as  mica. 

Foliation.  Under  still  stronger  pressure  the  whole  structure 
of  the  rock  is  altered.  The  minerals  of  which  it  is  composed, 
and  the  new  miner- 
als which  develop 
by  heat  and  pres- 
sure, arrange  them- 
selves along  planes 
of  cleavage  or  of 
shear  in  rudely  par- 
allel leaves,  or  folia. 
Of  this  structure, 
called  foliation,  we 
may  distinguish  two 
types, —  a  coarser 
f  eld  spathic  type, 
and  a  fine  type  in 
which  other  miner- 
als than  feldspar 
predominate. 

Gneiss  is  the 

FIG.  255.   A  Foliated  Rock 
general  name  under 

which  are  comprised  coarsely  foliated  rocks  banded  with  irregu- 
lar layers  of  feldspar  and  other  minerals.  The  gneisses  appear 
to  be  due  in  many  cases  to  the  crushing  and  shearing  of  deep- 
seated  igneous  rocks,  such  as  granite  and  gabbro. 

The  crystalline  schists,  representing  the  finer  types  of  folia- 
tion, consist  of  thin,  parallel,  crystalline  leaves,  which  are  often 
remarkably  crumpled.  These  folia  can  be  distinguished  from 
the  laminae  of  sedimentary  rocks  by  their  lenticular  form  and 


284  THE  ELEMENTS  OF  GEOLOGY 

lack  of  continuity,  and  especially  by  the  fact  that  they  consist 
of  platy,  crystalline  grains,  and  not  of  particles  rounded  by 
wear. 

Mica  schist,  the  most  common  of  schists,  and  in  fact  of  all  metamor- 
phic  rocks,  is  composed  of  mica  and  quartz  in  alternating  wavy  folia. 
All  gradations  between  it  and  phyllite  may  be  traced,  and  in  many 
cases  we  may  prove  it  due  to  the  metarnorphism  of  slates  and  shales. 
It  is  widespread  in  New  England  and  along  the  eastern  side  of  the 
Appalachians.  Talc  schist  consists  of  quartz  and  talc,  a  light-colored 
magnesian  mineral  of  greasy  feel,  and  so  soft  that  it  can  be  scratched 
with  the  thumb  nail. 

Hornblende  schist,  resulting  in  many  cases  from  the  foliation  of  basic 
igneous  rocks,  is  made  of  folia  of  hornblende  alternating  with  bands 
of  quartz  and  feldspar.  Hornblende  schist  is  common  over  large  areas 
in  the  Lake  Superior  region. 

Quartz  schist  is  produced  from  quartzite  by  the  development  of  fine 
folia  of  mica  along  planes  of  shear.  All  gradations  may  be  found 
between  it  and  unfoliated  quartzite  on  the  one  hand  and  mica  schist  on 
the  other. 

Under  the  resistless  pressure  of  crustal  movements  almost  any  rocks, 
sandstones,  shales,  lavas  of  all  kinds,  granites,  diorites,  and  gabbros 
may  be  metamorphosed  into  schists  by  crushing  and  shearing.  Lime- 
stones, however,  are  metamorphosed  by  pressure  into  marble,  the  grains 
of  carbonate  of  lime  recrystallizing  freely  to  interlocking  crystals  of 
calcite. 

These  few  examples  must  suffice  of  the  great  class  of  meta- 
morphic  rocks.  As  we  have  seen,  they  owe  their  origin  to  the 
alteration  of  both  of  the  other  classes  of  rocks  —  the  sedimentary 
and  the  igneous  —  by.  heat  and  pressure,  assisted  usually  by 
the  presence  of  water.  The  fact  of  change  is  seen  in  their  hard- 
ness and  cementation,  their  more  or  less  complete  recrystalli- 
zation,  and  their  foliation ;  but  the  change  is  often  so  complete 
that  no  trace  of  their  original  structure  and  mineral  composi- 
tion remains  to  tell  whether  the  rocks  from  which  they  were 
derived  were  sedimentary  or  igneous,  or  to  what  variety  of  either 
of  these  classes  they  belonged. 


285 


286 


THE   ELEMENTS  OF  GEOLOGY 


In  many  cases,  however,  the  early  history  of  a  metamorpliic 
rock  can  be  deciphered.  Fossils  not  wholly  obliterated  may 
prove  it  originally  water-laid.  Schists  may  contain  rolled-out 
pebbles,  showing  their  derivation  from  a  conglomerate.  Dikes 
of  igneous  rocks  may  be  followed  into  a  region  where  they  have 
been  foliated  by  pressure.  The  most  thoroughly  metamorphosed 
rocks  may  sometimes  be  traced  out  into  unaltered  sedimentary 
or  igneous  rocks,  or  among  them  may  be  found  patches  of  little 

change  where  their 
history  maybe  read. 
Metamorphism 
is  most  common 
among  rocks  of  the 
earlier  geological 
ages,  and  most  rare 
among  rocks  of 
recent  formation. 
No  doubt  it  is  now 
in  progress  where 

deep-buried  sedi- 
FIG.  257.   Quartz  Veins  in  Slate 

ments   are  invaded 

by  heat  either  from  intrusive  igneous  masses  or  from  the  earth's 
interior,  or  are  suffering  slow  deformation  under  the  thrust  of 
mountain-making  forces. 

Suggest  how  rocks  now  in  process  of  metamorphism  may  sometimes 
be  exposed  to  view.  Why  do  metamorphic  rocks  appear  on  the  surface 
to-day? 

MINERAL  VEINS 

In  regions  of  folded  and  broken  rocks  fissures  are  frequently 
found  to  be  filled  with  sheets  of  crystalline  minerals  deposited 
from  solution  by  underground  water,  and  fissures  thus  filled  are 
known  as  mineral  veins.  Much  of  the  importance  of  mineral 
veins  is  due  to  the  fact  that  they  are  often  metalliferous, 


METAMORPHISM  AND  MINERAL  VEINS  287 

carrying  valuable  native  metals  and  metallic  ores  disseminated 
in  fine  particles,  in  strings,  and  sometimes  in  large  masses  in  the 
midst  of  the  valueless  nonmetallic  minerals  which  make  up 
what  is  known  as  the  vein  stone. 

The  most  common  vein  stones  are  quartz  and  calcite.  Fluorite  (cal- 
cium fluoride),  a  mineral  harder  than  calcite  and  crystallizing  in  cubes 
of  various  colors,  and  barite  (barium  sulphate),  a  heavy  white  mineral, 
are  abundant  in  many  veins. 

The  gold-bearing  quartz  veins  of  California  traverse  the  metamor- 
phic  slates  of  the  Sierra  Nevada  Mountains.  Below  the  zone  of  solution 
(p.  45)  these  veins  consist  of  a  vein  stone  of  quartz  mingled  with 
pyrite  (p.  13),  the  latter  containing  threads  and  grains  of  native  gold. 
But  to  the  depth  of  about 
fifty  feet  from  the  surface 
the  pyrite  of  the  vein  has 
been  dissolved,  leaving  a 

rusty,  cellular  quartz  with 

FIG.  258.   Placer  Deposits  in  California 
grains  of  the  insoluble  gold 

scattered  through  it.  9,  gold-bearing  gravels  in  present  river  beds ;  g>, 

0  .  ancient  gold-bearing  river  gravels ;  a,  a,  lava 

The   placer    deposits    of         flows  capping  table  mountains ;  s,  slate.  Draw 

California  and  other         a  diagram  showing  by  dotted  lines  conditions 

regions     are    gold-bearing         before  the  lava  flows  occurred.   What  changes 

have  since  taken  place  ? 
deposits  of  gravel  and  sand 

in  river  beds.  The  heavy  gold  is  apt  to  be  found  mostly  near  or  upon  the 
solid  rock,  and  its  grains,  like  those  of  the  sand,  are  always  rounded. 
How  the  gold  came  in  the  placers  we  may  leave  the  pupil  to  suggest. 

Copper  is  found  in  a  number  of  ores,  and  also  in  the  native 
metal.  Below  the  zone  of  surface  changes  the  ore  of  a  cop- 
per vein  is  often  a  double  sulphide  of  iron  and  copper  called 
clialcopyrite,  a  mineral  softer  than  pyrite  —  it  can  easily  be 
scratched  with  a  knife  —  arid  deeper  yellow  in  color.  For  sev- 
eral score  of  feet  below  the  ground  the  vein  may  consist  of 
rusty  quartz  from  which  the  metallic  ores  have  been  dissolved ; 
but  at  the  base  of  the  zone  of  solution  we  may  find  exceedingly 
rich  deposits  of  copper  ores,  —  copper  sulphides,  red  and  black 
copper  oxides,  and  green  and  blue  copper  carbonates,  which 


288         THE  ELEMENTS  OF  GEOLOGY 

have  clearly  been  brought  down  in  solution  from  the  leached 
upper  portion  of  the  vein. 

Origin  of  mineral  veins.  Both  vein  stones  and  ores  have  been 
deposited  slowly  from  solution  in  water,  much  as  crystals  of  salt 
are  deposited  on  the  sides  of  a  jar  of  saturated  brine.  In  our 
study  of  underground  water  we  learned  that  it  is  everywhere 
circulating  through  the  permeable  rocks  of  the  crust,  descend- 
ing to  profound  depths  under  the  action  of  gravity  and  again 
driven  to  the  surface  by  hydrostatic  pressure.  Now  fissures, 
wherever  they  occur,  form  the  trunk  channels  of  the  under- 
ground circulation.  Water  descends  from  the  surface  along 
these  rifts ;  it  moves  laterally  from  either  side  to  the  fissure 
plane,  just  as  ground  water  seeps  through  the  surrounding  rocks 
from  every  direction  to  a  well ;  and  it  ascends  through  these 
natural  water  ways  as  in  an  artesian  well,  whenever  they  inter- 
sect an  aquifer  in  which  water  is  under  hydrostatic  pressure. 

The  waters  which  deposit  vein  stones  and  ores  are  commonly 
hot,  and  in  many  cases  they  have  derived  their  heat  from  intru- 
sions of  igneous  rock  still  uncooled  within  the  crust.  The  sol- 
vent power  of  the  water  is  thus  greatly  increased,  and  it  takes 
up  into  solution  various  substances  from  the  igneous  and  sedi- 
mentary rocks  which  it  traverses.  For  various  reasons  these  sub- 
stances are  deposited  in  the  vein  as  ores  and  vein  stones.  On 
rising  through  the  fissure  the  water  cools  and  loses  pressure,  and 
its  capacity  to  hold  minerals  in  solution  is  therefore  lessened. 
Besides,  as  different  currents  meet  in  the  fissure,  some  ascend- 
ing, some  descending,  and  some  coming  in  from  the  sides,  the 
chemical  reaction  of  these  various  weak  solutions  upon  one 
another  and  upon  the  walls  of  the  vein  precipitates  the  minerals 
of  vein  stuffs  and  ores. 

As  an  illustration  of  the  method  of  vein  deposits  we  may  cite  the  case 
of  a  wooden  box  pipe  used  in  the  Comstock  mines,  Nevada,  to  carry  the 
hot  water  of  the  mine  from  one  level  to  another,  which  in  ten  years 
was  lined  with  calcium  carbonate  more  than  half  an  inch  thick. 


METAMORPHISM  AND  MINERAL  VEINS  289 

The  Steamboat  Springs,  Nevada,  furnish  examples  of  mineral  veins 
in  process  of  formation.  The  steaming  water  rises  through  fissures  in 
volcanic  rocks  and  is  now  depositing  in  the  rifts  a  vein  stone  of  quartz, 
with  metallic  ores  of  iron,  mercury,  lead,  and  other  metals. 

Reconcentration.  Near  the  base  of  the  zone  of  solution  veins 
are  often  stored  with  exceptionally  large  and  valuable  ore 
deposits.  This  local  enrichment  of  the  vein  is  due  to  the  recon- 
centration  of  its  metalliferous  ores.  As  the  surface  of  the  land 


FIG.  259.  Reconcentration  of  Ores  in  Mineral  Veins 

A,  original  vein ;  B,  same  after  reconcentration ;  v,  mineral  vein ;  s,  sur- 
face of  ground  (dotted  line,  former  surface  of  the  ground) ;  sp,  spring; 
o,  vein  leached  of  ores  by  descending  waters  in  zone  of  solution; 
TO,  rich  ore  deposits  reconcentrated  from  above ;  n,  unchanged  portion 
of  vein 

is  slowly  lowered  by  weathering  and  running  water,  the  zone  of 
solution  is  lowered  at  an  equal  rate  and  encroaches  constantly 
on  the  zone  of  cementation.  The  minerals  of  veins  are  therefore 
constantly  being  dissolved  along  their  upper  portions  and  carried 
down  the  fissures  by  ground  water  to  lower  levels,  where  they 
are  redeposited. 

Many  of  the  richest  ore  deposits  are  thus  due  to  successive 
concentrations :  the  ores  were  leached  originally  from  the  rocks 


290         THE  ELEMENTS  OF  GEOLOGY 

to  a  large  extent  by  laterally  seeping  waters ;  they  were  concen- 
trated in  the  ore  deposits  of  the  vein  chiefly  by  ascending  cur- 
rents; they  have  been  reconcentrated  by  descending  waters  in 
the  way  just  mentioned. 

The  original  source  of  the  metals.  It  is  to  the  igneous  rocks 
that  we  may  look  for  the  original  source  of  the  metals  of  veins. 
Lavas  contain  minute  percentages  of  various  metallic  com- 
pounds, and  no  doubt  this  was  the  case  also  with  the  igneous 
rocks  which  formed  the  original  earth  crust.  Intrusive  masses 
of  molten  rock,  as  they  cool  and  solidify,  may  segregate  rich 
mineral  deposits,  such  as  magnetic  iron  ore.  They  give  off  vast 
quantities  of  steam  and  other  gases,  which  carry  out  metallic 
ores  in  solution  and  deposit  them  in  the  surrounding  strata. 
In  fissures  these  ores  are  carried  farther  by  hot  ascending 
waters  and  laid  down  in  mineral  veins.  By  the  erosion  of  the 
igneous  rocks  the  metals  have  been  distributed  among  sedi- 
mentary strata,  and  even  the  sea  has  taken  into  solution  an 
appreciable  amount  of  gold  and  other  metals,  but  in  this 
widely  diffused  condition  they  are  wholly  useless  to  man. 
The  concentration  which  has  made  them  available  is  due  to 
the  interaction  of  many  agencies.  Earth  movements  fracturing 
deeply  the  rocks  of  the  crust,  the  intrusion  of  heated  masses, 
the  circulation  of  underground  waters,  have  all  cooperated  in 
the  concentration  of  the  metals  of  mineral  veins. 

While  fissure  veins  are  the  most  important  of  mineral  veins,  the 
latter  term  is  applied  also  to  any  water  way  which  has  been  filled  by  simi- 
lar deposits  from  solution.  Thus,  in  soluble  rocks,  such  as  limestones, 
joints  enlarged  by  percolating  water  are  sometimes  filled  with  metallif- 
erous deposits,  as,  for  example,  the  lead  and  zinc  deposits  of  the  upper 
Mississippi  valley.  Even  a  porous  aquifer  may  be  made  the  seat  of 
mineral  deposits,  as  in  the  case  of  some  copper-and-silver-bearing  sand- 
stones of  New  Mexico. 


Key  to  Colors  and  Letters 

r ,Cai 

I I  (W.of  the  Great  Plains)  C 

|  |  Devonian   I) 

— 1  Silurian  and 

J I  Ordovk-iaii   O 

Cambrian     e 


f 1  Quaternary  I 1  Carboniferous 

I I  (W.of  the  Rocky  Mts.)Q 

Tertiary  T 
Cretaceous  K 


rnrasalc  and 

['i-iussu-     J 


— "—i  Peniisylvaiilan 

I \t      ' 


land  Permian   P 


Pre-Cambrian   A 


I  I  Mississlpplan   M  I  I  Igneous   I 


O 


FIG.  260.   Geological  Map 


GULF    OF   MEXICO 


3d  States  and  Part  of  Canada 


PAET  III 

HISTORICAL  GEOLOGY 

CHAPTER  XIV 
THE  GEOLOGICAL  RECORD 

What  a  formation  records.  We  have  already  learned  that 
each  individual  body  of  stratified  rock,  or  formation,  constitutes 
a  record  of  the  time  when  it  was  laid.  The  structure  and  the 
character  of  the  sediments  of  each  formation  tell  whether  the 
area  was  land  or  sea  at  the  time  when  they  were  spread ;  and 
if  the  former,  whether  the  land  was  river  plain,  or  lake  bed, 
or  was  covered  with  wind-blown  sands,  or  by  the  deposits  of 
an  ice  sheet.  If  the  sediments  are  marine,  we  may  know  also 
whether  they  were  laid  in  shoal  water  near  the  shore  or  in 
deeper  water  out  at  sea,  and  whether  during  a  period  of  emer- 
gence, or  during  a  period  of  subsidence  when  the  sea  transgressed 
the  land.  By  the  same  means  each  formation  records  the  stage 
in  the  cycle  of  erosion  of  the  land  mass  from  which  its  sediments 
were  derived  (p.  185).  An  unconformity  between  two  marine 
formations  records  the  fact  that  between  the  periods  when 
they  were  deposited  in  the  sea  the  area  emerged  as  land  and 
suffered  erosion  (p.  227).  The  attitude  and  structure  of  the 
strata  tell  also  of  the  foldings  and  fractures,  the  deformation 
and  the  metamorphism,  which  they  have  suffered;  and  the 
igneous  rocks  associated  with  them  as  lava  flows  and  igneous 
intrusions  add  other  details  to  the  story.  Each  formation  is 
thus  a  separate  local  chapter  in  the  geological  history  of  the 

291 


292  THE  ELEMENTS  OF  GEOLOGY 

earth,  and  its  strata  are  its  leaves.  It  contains  an  authentic 
record  of  the  physical  conditions  —  the  geography  —  of  the 
time  and  place  when  and  where  its  sediments  were  laid. 

Past  cycles  of  erosion.  These  chapters  in  the  history  of  the 
planet  are  very  numerous,  although  much  of  the  record  has  been 
destroyed  in  various  ways.  A  succession  of  different  formations 
is  usually  seen  in  any  considerable  section  of  the  crust,  such  as 
a  deep  canyon  or  where  the  edges  of  upturned  strata  are  exposed 
to  view  on  the  flanks  of  mountain  ranges ;  and  in  any  extensive 
area,  such  as  a  state  of  the  Union  or  a  province  of  Canada,  the 
number  of  formations  outcropping  on  the  surface  is  large. 

It  is  thus  learned  that  our  present  continent  is  made  up  for 
the  most  part  of  old  continental  deltas.  Some,  recently  emerged 
as  the  strata  of  young  coastal  plains,  are  the  records  of  recent 
cycles  of  erosion ;  while  others  were  deposited  in  the  early  his- 
tory of  the  earth,  and  in  many  instances  have  been  crumpled 
into  mountains,  which  afterwards  were  leveled  to  their  bases 
and  lowered  beneath  the  sea  to  receive  a  cover  of  later  sedi- 
ments before  they  were  again  uplifted  to  form  land. 

The  cycle  of  erosion  now  in  progress  and  recorded  in  the 
layers  of  stratified  rock  being  spread  beneath  the  sea  in  conti- 
nental deltas  has  therefore  been  preceded  by  many  similar  cycles. 
Again  and  again  movements  of  the  crust  have  brought  to  an 
end  one  cycle  —  sometimes  when  only  well  under  way,  and 
sometimes  when  drawing  toward  its  close  —  and  have  begun 
another.  Again  and  again  they  have  added  to  the  land  areas 
which  before  were  sea,  with  all  their  deposition  records  of 
earlier  cycles,  or  have  lowered  areas  of  land  beneath  the  sea  to 
receive  new  sediments. 

The  age  of  the  earth.  The  thickness  of  the  stratified  rocks 
now  exposed  upon  the  eroded  surface  of  the  continents  is  very 
great.  In  the  Appalachian  region  the  strata  are  seven  or  eight 
miles  thick,  and  still  greater  thicknesses  have  been  measured  in 
several  other  mountain  ranges.  The  aggregate  thickness  of  all 


THE  GEOLOGICAL  RECORD  293 

the  formations  of  the  stratified  rocks  of  the  earth's  crust,  giving 
to  each  formation  its  maximum  thickness  wherever  found, 
amounts  to  not  less  than  forty  miles.  Knowing  how  slowly 
sediments  accumulate  upon  the  sea  floor  (p.  184),  we  must 
believe  that  the  successive  cycles  which  the  earth  has  seen 
stretch  back  into  a  past  almost  inconceivably  remote,  and 
measure  tens  of  millions  and  perhaps  even  hundreds  of  millions 
of  years. 

How  the  formations  are  correlated  and  the  geological  record 
made  up.  Arranged  in  the  order  of  then-  succession,  the  forma- 
tions of  the  earth's  crust  would  constitute  a  connected  record  in 
which  the  geological  history  of  the  planet  may  be  read,  and 
therefore  known  as  the  geological  record.  But  to  arrange  the 
formations  in  their  natural  order  is  not  an  easy  task.  A  com- 
plete set  of  the  volumes  of  the  record  is  to  be  found  in  no  single 
region.  Their  leaves  and  chapters  are  scattered  over  the  land 
surface  of  the  globe.  In  one  area  certain  chapters  may  be 
found,  though  perhaps  with  many  missing  leaves,  and  with  inter- 
vening chapters  wanting,  and  these  absent  parts  perhaps  can  be 
supplied  only  after  long  search  through  many  other  regions. 

Adjacent  strata  in  any  region  are  arranged  according  to  the 
law  of  superposition,  i.e.  any  stratum  is  younger  than  that  on 
which  it  was  deposited,  just  as  in  a  pile  of  paper,  any  sheet  was 
laid  later  than  that  on  which  it  rests.  Where  rocks  have  been 
disturbed,  their  original  attitude  must  be  determined  before  the 
law  can  be  applied.  -  Nor  can  the  law  of  superposition  be  used 
in  identifying  and  comparing  the  strata  of  different  regions 
where  the  formations  cannot  be  traced  continuously  from  one 
region  to  the  other. 

The  formations  of  different  regions  are  arranged  in  their  true 
order  by  the  law  of  included  organisms ;  i.e.  formations,  how- 
ever widely  separated,  which  contain  a  similar  assemblage  of 
fossils  are  equivalent  and  belong  to  the  same  division  of  geo- 
logical time. 


294  THE  ELEMENTS  OF  GEOLOGY 

The  correlation  of  formations  by  means  of  fossils  may  be  explained 
by  the  formations  now  being  deposited  about  the  north  Atlantic.  Litho- 
logically  they  are  extremely  various.  On  the  continental  shelf  of  North 
America  limestones  of  different  kinds  are  forming  oft'  Florida,  and  sand- 
stones and  shales  from  Georgia  northward.  Separated  from  them  by 
the  deep  Atlantic  oozes  are  other  sedimentary  deposits  now  accumulat- 
ing along  the  west  coast  of  Europe.  If  now  all  these  offshore  formations 
were  raised  to  open  air,  how  could  they  be  correlated  ?  Surely  not  by 
lithological  likeness,  for  in  this  respect  they  would  be  quite  diverse.  All 
would  be  similar,  however,  in  the  fossils  which  they  contain.  Some 
fossil  species  would  be  identical  in  all  these  formations  and  others 
would  be  closely  allied.  Making  all  due  allowance  for  differences  in 
species  due  to  local  differences  in  climate  and  other  physical  causes,  it 
would  still  be  plain  that  plants  and  animals  so  similar  lived  at  the  same 
period  of  time,  and  that  the  formations  in  which  their  remains  were 
imbedded  were  contemporaneous  in  a  broad  way.  The  presence  of  the 
bones  of  whales  and  other  marine  mammals  would  prove  that  the  strata 
were  laid  after  the  appearance  of  mammals  upon  earth,  and  imbedded 
relics  of  man  would  give  a  still  closer  approximation  to  their  age.  In 
the  same  way  we  correlate  the  earlier  geological  formations. 

For  example,  in  1902  there  were  collected  the  first  fossils  ever  found 
on  the  antarctic  continent.  Among  the  dozen  specimens  obtained  were 
some  fossil  ammonites  (a  family  of  chambered  shells)  of  genera  which 
are  found  on  other  continents  in  certain  formations  classified  as  the 
Cretaceous  system,  and  which  occur  neither  above  these  formations  nor 
below  them.  On  the  basis  of  these  few  fossils  we  may  be  confident  that 
the  strata  in  which  they  were  found  in  the  antarctic  region  were  laid 
in  the  same  period  of  geologic  time  as  were  the  Cretaceous  rocks  of  the 
United  States  and  Canada. 

The  record  as  a  time  scale.  By  means  of  the  law  of  included 
organisms  and  the  law  of  superposition  the  formations  of  differ- 
ent countries  and  continents  are  correlated  and  arranged  in 
their  natural  order.  When  the  geological  record  is  thus  obtained 
it  may  be  used  as  a  universal  time  scale  for  geological  history. 
Geological  time  is  separated  into  divisions  corresponding  to  the 
times  during  which  the  successive  formations  were  laid.  The 
largest  assemblages  of  formations  are  known  as  groups/ while  the 


THE  GEOLOGICAL  RECORD  295 

corresponding  divisions  of  time  are  known  as  eras.  Groups 
are  subdivided  into  systems,  and  systems  into  series.  Series 
are  divided  into  stages  and  substages,  —  subdivisions  which 
do  not  concern  us  in  this  brief  treatise.  The  corresponding 
divisions  of  time  are  given  in  the  following  table. 

Strata  Time 

Group  Era 

System  Period 

Series  Epoch 

The  geologist  is  now  prepared  to  read  the  physical  history  — 
the  geographical  development  —  of  any  country  or  of  any  conti- 
nent by  means  of  its  formations,  when  he  has  given  each  for- 
mation its  true  place  in  the  geological  record  as  a  time  scale. 

The  following  chart  exhibits  the  main  divisions  of  the  record, 
the  name  given  to  each  being  given  also  to  the  corresponding 
time  division.  Thus  we  speak  of  the  Cambrian  system,  mean- 
ing a  certain  succession  of  formations  which  are  classified 
together  because  of  broad  resemblances  in  their  included  organ- 
isms ;  and  of  the  Cambrian  period,  meaning  the  time  during 
which  these  rocks  were  deposited. 

Group  and  Era   ,  System  and  Period  Series  and  Epoch 

f  Recent 
Quaternary    ....       j  pleistocene 

Cenozoic    ....      *  f  Pliocene    ' 

I  Tertiary <j  Miocene 

f-  Cretaceous  L  Eocene 
Mesozoic    .    V    .     .      J  Jurassic 

L  Triassic  r  Permian 

Carboniferous  .     .     .      <  Pennsylvanian 

Devonian  L  Mississippian 


Paleozoic 


Silurian 


Ordovician 
^  Cambrian 
Proterozoic  Era,  Algonkian  Group 
Archeozoic  Era,  Archean  Group 


296         THE  ELEMENTS  OF  GEOLOGY 


FOSSILS   AND   WHAT   THEY   TEACH 

The  geological  formations  contain  a  record  still  more  impor- 
tant than  that  of  the  geographical  development  of  the  conti- 
nents ;  the  fossils  imbedded  in  the  rocks  of  each  formation  tell 
of  the  kinds  of  animals  and  plants  which  inhabited  the  earth  at 
that  tune,  and  from  these  fossils  we  are  therefore  able  to  con- 
struct the  history  of  life  upon  the  earth. 

Fossils.  These  remains  of  organisms  are  found  in  the  strata  in  all 
degrees  of  perfection,  from  trails  and  tracks  and  fragmentary  impres- 
sions, to  perfectly  preserved  shells,  wood,  bones,  and  complete  skeletons. 
As  a  rule,  it  is  only  the  hard  parts  of  animals  and  plants  which  have 
left  any  traces  in  the  rocks.  Sometimes  the  original  hard  substance  is 
preserved,  but  more  often  it  has  been  replaced  by  some  less  soluble 
material.  Petrifaction,  as  this  process  of  slow  replacement  is  called,  is 
often  carried  on  in  the  most  exquisite  detail.  When  wood,  for  example, 
is  undergoing  petrifaction,  the  woody  tissue  may  be  replaced,  particle 
by  particle,  by  silica  in  solution  through  the  action  of  underground 
waters,  even  the  microscopic  structures  of  the  wood  being  perfectly 
reproduced.  In  shells  originally  made  of  aragonite,  a  crystalline  form 
of  carbonate  of  lirne,  that  mineral  is  usually  replaced  by  calcite,  a  more 
stable  form  of  the  same  substance.  The  most  common  petrifying 
materials  are  calcite,  silica,  and  pyrite  (p.  13). 

Often  the  organic  substance  has  neither  been 
preserved  nor  replaced,  but  the  form  has  been 
retained  by  means  of  molds  and  casts.    Permanent 
impressions,  or  molds,  may  be  made  in  sediments 
not  only  by  the  hard  parts  of  organisms,  but  also 
FIG.  261.   Section  of     by  such  soft  and  perishable  parts  as  the  leaves  of 
Cast  and  Mold  of     plants,  and,  in  the  rarest  instances,  by  the  skin  of 
animals  and  the  feathers  of  birds.    In  fine-grained 

a,  shell;    6,  mold  of     limestones  even  the  imprints  of  jellyfish  have  been 

exterior;   c,  cast  of         .    .       , 

interior  retained. 

The  different  kinds  of  molds  and  casts  may  be 

illustrated  by  means  of  a  clam  shell  and  some  moist  clay,  the  latter 
representing  the  sediments  in  which  the  remains  of  animals  and  plants 
are  entombed.  Imbedding  the  shell  in  the  clay  and  allowing  the  clay 


THE  GEOLOGICAL  RECORD  297 

to  harden,  we  have  a  mold  of  the  exterior  of  the  shell,  as  is  seen  on 
cutting  the  clay  matrix  in  two  and  removing  the  shell  from  it.  Filling 
this  mold  with  clay  of  different  color,  we  obtain  a  cast  of  the  exterior, 
which  represents  accurately  the  original  form  and  surface  markings  of 
the  shell.  In  nature,  shells  and  other  relics  of  animals  or  plants  are 
often  removed  by  being  dissolved  by  percolating  waters,  and  the  molds 
are  either  filled  with  sediments  or  with  minerals  deposited  from 
solution. 

Where  the  fossil  is  hollow,  a  cast  of  the  interior  is  made  in  the  same 
way.  Interior  casts  of  shells  reproduce  any  markings  on  the  inside  of 
the  valves,  and  casts  of  the  interior  of  the  skulls  of  ancient  vertebrates 
show  the  form  and  size  of  their  brains. 

Imperfection  of  the  life  record.  At  the  present  time  only  the 
smallest  fraction  of  the  life  on  earth  ever  gets  entombed  in 
rocks  now  forming.  In  the  forest  great  fallen  tree  trunks,  as  well 
as  dead  leaves,  decay,  and  only  add  a  little  to  the  layer  of  dark 
vegetable  mold  from  which  they  grew.  The  bones  of  land 
animals  are,  for  the  most  part,  left  unburied  on  the  surface  and 
are  soon  destroyed  by  chemical  agencies.  Even  where,  as  in  the 
swamps  of  river  flood  plains  and  in  other  bogs,  there  are  pre- 
served the  remains  of  plants,  and  sometimes  insects,  together 
with  the  bones  of  some  animal  drowned  or  mired,  in  most  cases 
these  swamp  and  bog  deposits  are  sooner  or  later  destroyed  by 
the  shifting  channels  of  the  stream  or  by  the  general  erosion 
of  the  land. 

In  the  sea  the  conditions  for  preservation  are  more  favorable 
than  on  land;  yet  even  here  the  proportion  of  animals  and 
plants  whose  hard  parts  are  fossilized  is  very  small  compared 
with  those  which  either  totally  decay  before  they  are  buried  in 
slowly  accumulating  sediments  or  are  ground  to  powder  by 
waves  and  currents. 

We  may  infer  that  during  each  period  of  the  past,  as  at 
the  present,  only  a  very  insignificant  fraction  of  the  innumer- 
able organisms  of  sea  and  land  escaped  destruction  and  left  in 
continental  and  oceanic  deposits  permanent  records  of  their 


298         THE  ELEMENTS  OF  GEOLOGY 

existence.  Scanty  as  these  original  life  records  must  have 
been,  they  have  been  largely  destroyed  by  metamorphism  of 
the  rocks  in  which  they  were  imbedded,  by  solution  in  un- 
derground waters,  and  by  the  vast  denudation  under  which 
the  sediments  of  earlier  periods  have  been  eroded  to  furnish 
materials  for  the  sedimentary  records  of  later  times.  Moreover, 
very  much  of  what  has  escaped  destruction  still  remains  undis- 
covered. The  immense  bulk  of  the  stratified  rocks  is  buried 
and  inaccessible,  and  the  records  of  the  past  which  it  contains 
can  never  be  known.  Comparatively  few  outcrops  have  been 
thoroughly  searched  for  fossils.  Although  new  species  are  con- 
stantly being  discovered,  each  discovery  may  be  considered  as 
the  outcome  of  a  series  of  happy  accidents,  —  that  the  remains  of 
individuals  of  this  particular  species  happened  to  be  imbedded 
and  fossilized,  that  they  happened  to  escape  destruction  during 
long  ages,  and  that  they  happened  to  be  exposed  and  found. 

Some  inferences  from  the  records  of  the  history  of  life  upon 
the  planet.  Meager  as  are  these  records,  they  set  forth  plainly 
some  important  truths  which  we  will  now  briefly  mention. 

1.  Each  series  of  the  stratified  rocks,  except  the  very  deepest, 
contains  vestiges  of  life.    Hence  the  earth  was  tenanted  by  living 
creatures  for  an  uncalculated  length  of  time  before  human  his- 
tory began. 

2.  Life  on  the  earth  has  been  ever  changing.    The  youngest 
strata  hold  the   remains   of    existing  species   of   animals   and 
plants  and  those  of  species  and  varieties 'closely  allied  to  them. 
Strata  somewhat  older  contain  fewer  existing  species,  and  in 
strata  of  a  still  earlier,  but  by  no  means  an  ancient  epoch, 
no  existing  species  are  to  be  found ;  the  species  of  that  epoch 
and  of  previous  epochs  have  vanished  from  the  living  world. 
During  all  geological  time  since  life  began  on  earth  old  species 
have  constantly  become  extinct  and  with  them  the  genera  and 
families  to  which  they  belong,  and  other  species,  genera,  and 
families  have  replaced  them.    The  fossils  of  each  formation 


THE  GEOLOGICAL  RECORD  299 

differ  on  the  whole  from  those  of  every  other.  The  assemblage 
of  animals  and  plants  (the  fauna-flora)  of  each  epoch  differs 
from  that  of  every  other  epoch. 

In  many  cases  the  extinction  of  a  type  has  been  gradual ;  in 
other  instances  apparently  abrupt.  There  is  no  evidence  that 
any  organism  once  become  extinct  has  ever  reappeared.  The 
duration  of  a  species  in  tune,  or  its  "vertical  range"  through 
the  strata,  varies  greatly.  Some  species  are  limited  to  a  stratum 
a  few  feet  in  thickness;  some  may  range  through  an  entire 
formation  and  be  found  but  little  modified  in  still  higher  beds. 
A  formation  may  thus  often  be  divided  into  zones,  each  char- 
acterized by  its  own  peculiar  species.  As  a  rule,  the  simpler 
organisms  have  a  longer  duration  as  species,  though  not  as  in- 
dividuals, than  the  more  complex. 

3.  The    larger   zoological    and  botanical  groupings   survive 
longer  than  the  smaller.    Species  are  so  short-lived  that  a  single 
geological  epoch  may  be  marked  by  several  more  or  less  com- 
plete extinctions  of  the  species  of  its  fauna-flora  and  their 
replacement  by  other  species.    A  genus  continues  with  new 
species  after  all  the  species  with  which  it  began  have  become 
extinct.    Families  survive  genera,  and  orders  families.    Classes 
are  so  long-lived  that  most  of  those  which  are  known  from  the 
earliest  formations  are  represented  by  living  forms,  and  no  sub- 
kingdom  has  ever  become  extinct. 

Thus,  to  take  an  example  from  the  stony  corals,  —  the  Zoantharia, 
—  the  particular  characters  which  constituted  a  certain  species  —  Favo- 
sites  niagarensis.  —  of  the  order  are  confined  to  the  Niagara  series.  Its 
generic  characters  appeared  in  other  species  earlier  in  the  Silurian  and 
continued  through  the  Devonian.  Its  family  characters,  represented  in 
different  genera  and  species,  range  from  the  Ordovician  to  the  close  of 
the  Paleozoic ;  while  the  characters  which  it  shares  with  all  its  order,  the 
Zoantharia,  began  in  the  Cambrian  and  are  found  in  living  species. 

4.  The  change  in  organisms  has  been  gradual.    The  fossils 
of  each  life  zone  and  of  each  formation  of  a  conformable  series 


300  THE  ELEMENTS  OF   GEOLOGY 

closely  resemble,  with  some  explainable  exceptions,  those  of  the 
beds  immediately  above  and  below.  The  animals  and  plants 
which  tenanted  the  earth  during  any  geological  epoch  are  so 
closely  related  to  those  of  the  preceding  and  the  succeeding 
epochs  that  we  may  consider  them  to  be  the  descendants  of  the 
one  and  the  ancestors  of  the  other,  thus  accounting  for  the  resem- 
blance by  heredity.  It  is  therefore  believed  that  the  species  of 
animals  and  plants  now  living  on  the  earth  are  the  descendants 
of  the  species  whose  remains  we  find  entombed  in  the  rocks,  and 
that  the  chain  of  life  has  been  unbroken  since  its  beginning. 

5.  The  change  in  species  has  been  a  gradual  differentiation. 
Tracing  the  lines  of  descent  of  various  animals  and  plants  of 
the  present  backward  through  the  divisions  of  geologic  time,  we 
find  that  these  lines  of  descent  converge  and  unite  in  simpler 
and  still  simpler  types.    The  development  of  life  may  be  repre- 
sented by  a  tree  whose  trunk  is  found  in  the  earliest  ages  and 
whose  branches  spread  and  subdivide  to  the  growing  twigs  of 
present  species. 

6.  The  change  in  organisms  throughout  geologic  time  has  been 
a  progressive  change.    In  the  earliest  ages  the  only  animals  and 
plants  on  the  earth  were  lowly  forms,  simple  and  generalized  in 
structure ;  while  succeeding  ages  have  been  characterized  by 
the  introduction  of  types  more  and  more  specialized  and  com- 
plex, and  therefore  of  higher  rank  in  the  scale  of  being.    Thus 
the  Algonkian  contains  the  remains  of  only  the  humblest  forms 
of  the  invertebrates.    In  the  Cambrian,  Ordovician,  and  Silurian 
the  invertebrates  were  represented  in  all  their  subkingdoms  by 
a  varied  fauna.    In  the  Devonian,  fishes  —  the  lowest   of  the 
vertebrates  —  became  abundant.    Amphibians  made  their  entry 
on  the  stage  in  the  Carboniferous,  and  reptiles  came  to  rule  the 
world  in  the  Mesozoic.    Mammals  culminated  in  the  Tertiary 
in  strange  forms  which  became  more  and  more  like  those  of  the 
present  as  the  long  ages  of  that  era  rolled  on ;  and  latest  of  all 
appeared  the  noblest  product  of  the  creative  process,  man. 


THE  GEOLOGICAL  RECORD  301 

Just  as  growth  is  characteristic  of  the  individual  life,  so 
gradual,  progressive  change,  or  evolution,  has  characterized  the 
history  of  life  upon  the  planet.  The  evolution  of  the  organic 
kingdom  from  its  primitive  germinal  forms  to  the  complex  and 
highly  organized  fauna-flora  of  to-day  may  be  compared  to  the 
growth  of  some  noble  oak  as  it  rises  from  the  acorn,  spreading 
loftier  and  more  widely  extended  branches  as  it  grows. 

7.  While  higher  and  still  higher  types  have  continually  been 
evolved,  until  man,  the  highest  of  all,  appeared,  the  lower  and 
earlier  types   have  geffierally  persisted.    Some  which  reached 
their  culmination  early  in  the  history  of  the  earth  have  since 
changed  only  in  slight  adjustments  to  a  changing  environment. 
Thus  the  brachiopods,  a  type  of  shellfish,  have  made  no  prog- 
ress since  the  Paleozoic,  and  some  of  then-  earliest  known  genera 
are  represented  by  living  forms  hardly  to  be  distinguished  from 
their  ancient  ancestors.    The  lowest  and  earliest  branches  of  the 
tree  of  life  have  risen  to  no  higher  levels  since  they  reached 
their  climax  of  development  long  ago. 

8.  A  strange  parallel  has  been  found  to  exist  between  the 
evolution  of  organisms  and  the  development  of  the  individual. 
In  the  embryonic  stages  of  its  growth  the  individual  passes 
swiftly  through  the  successive  stages  through  which  its  ances- 
tors evolved  during  the  millions  of  years  of  geologic  time. 
The  development  of  the  individual  recapitulates  the  evolution 
of  the  race. 

The  frog  is  a  typical  amphibian.  As  a  tadpole  it  passes  through  a 
stage  identical  in  several  well-known  features  with  the  maturity  of 
fishes ;  as,  for  example,  its  aquatic  life,  the  tail  by  which  it  swims,  and 
the  gills  through  which  it  breathes.  It  is  a  fair  inference  that  the  tad- 
pole stage  in  the  life  history  of  the  frog  represents  a  stage  in  the  evolu- 
tion of  its  kind,  —  that  the  Amphibia  are  derived  from  fishlike  ancestral 
forms.  This  inference  is  amply  confirmed  in  the  geological  record; 
fishes  appeared  before  Amphibia  and  were  connected  with  them  by 
transitional  forms. 


302  THE  ELEMENTS  OF  GEOLOGY 

The  great  length  of  geologic  time  inferred  from  the  slow 
change  of  species.  Life  forms,  like  land  forms,  are  thus  subject 
to  change  under  the  influence  of  their  changing  environment  and 
of  forces  acting  from  within.  How  slowly  they  change  may  be 
seen  in  the  apparent  stability  of  existing  species.  In  the  lifetime 
of  the  observer  and  even  in  the  recorded  history  of  man,  species 
seem  as  stable  as  the  mountain  and  the  river.  But  life  forms 
and  land  forms  are  alike  variable,  both  in  nature  and  still  more 
under  the  shaping  hand  of  man.  As  man  has  modified  the  face 
of  the  earth  with  his  great  engineering  works,  so  he  has  pro- 
duced widely  different  varieties  of  many  kinds  of  domesticated 
plants  and  animals,  such  as  the  varieties  of  the  dog  and  the 
horse,  the  apple  and  the  rose,  which  may  be  regarded  in  some 
respects  as  new  species  in  the  making.  We  have  assumed  that 
land  forms  have  changed  in  the  past  under  the  influence  of 
forces  now  in  operation.  Assuming  also  that  life  forms  have 
always  changed  as  they  are  changing  at  present,  we  come  to 
realize  something  of  the  immensity  of  geologic  time  required 
for  the  evolution  of  life  from  its  earliest  lowly  forms  up  to  man. 

It  is  because  the  onward  march  of  life  has  taken  the  same 
general  course  the  world  over  that  we  are  able  to  use  it  as  a 
universal  time  scale  and  divide  geologic  time  into  ages  and 
minor  subdivisions  according  to  the  ruling  or  characteristic 
organisms  then  living  on  the  earth.  Thus,  since  vertebrates 
appeared,  we  have  in  succession  the  Age  of  Fishes,  the  Age  of 
Amphibians,  the  Age  of  Reptiles,  and  the  Age  of  Mammals. 

The  chart  given  on  page  295  is  thus  based  011  the  law  of 
superposition  and  the  law  of  the  evolution  of  organisms.  The 
first  law  gives  the  succession  of  the  formations  in  local  areas. 
The  fossils  which  they  contain  demonstrate  the  law  of  the  pro- 
gressive appearance  of  organisms,  and  by  means  of  this  law  the 
formations  of  different  countries  are  correlated  and  set  each  in 
its  place  in  a  universal  time  scale  and  grouped  together  accord- 
ing to  the  affinities  of  their  imbedded  organic  remains, 


THE  GEOLOGICAL  RECORD  303 

Geologic  time  divisions  compared  with  those  of  human  history.  We 
may  compare  the  division  of  geologic  time  into  eras,  periods,  and  other 
divisions  according  to  the  dominant  life  of  the  time,  to  the  ill-defined 
ages  into  which  human  history  is  divided  according  to  the  dominance  of 
some  nation,  ruler,  or  other  characteristic  feature.  Thus  we  speak  of  the 
Dark  Ages,  the  Age  of  Elizabeth,  and  the  Age  of  Electricity.  These  crude 
divisions  would  be  of  much  value  if,  as  in  the  case  of  geologic  time,  we 
had  no  exact  reckoning  of  human  history  by  years. 

And  as  the  course  of  human  history  has  flowed  in  an  unbroken 
stream  along  quiet  reaches  of  slow  change  and  through  periods  of  rapid 
change  and  revolution,  so  with  the  course  of  geologic  history.  Periods 
of  quiescence,  in  which  revolutionary  forces  are  perhaps  gathering  head, 
alternate  with  periods  of  comparatively  rapid  change  in  physical  geog- 
raphy and'  in  organisms,  when  new  and  higher  forms  appear  which  serve 
to  draw  the  boundary  line  of  new  epochs.  Nevertheless,  geological  his- 
tory is  a  continuous  progress;  its  periods  and  epochs  shade  into  one 
another  by  imperceptible  gradations,  and  all  our  subdivisions  must  needs 
be  vague  and  more  or  less  arbitrary. 

How  fossils  tell  of  the  geography  of  the  past.  Fossils  are 
used  not  only  as  a  record  of  the  development  of  life  upon  the 
earth,  but  also  in  testimony  to  the  physical  geography  of  past 
epochs.  They  indicate  whether  in  any  region  the  climate  was 
tropical,  temperate,  or  arctic.  Since  species  spread  slowly  from 
some  center  of  dispersion  where  they  originate  until  some  barrier 
limits  their  migration  farther,  the  occurrence  of  the  same  species 
in  rocks  of  the  same  system  in  different  countries  implies  the 
absence  of  such  barriers  at  the  period.  Thus  in  the  collection 
of  antarctic  fossils  referred  to  on  page  294  there  were  shallow- 
water  marine  shells  identical  in  species  with  Mesozoic  shells 
found  in  India  and  in  the  southern  extremity  of  South  America. 
Since  such  organisms  are  not  distributed  by  the  currents  of  the 
deep  sea  and  cannot  migrate  along  its  bottom,  we  infer  a  shal- 
low-water connection  in  Mesozoic  times  between  India,  South 
America,  and  the  antarctic  region.  Such  a  shallow-water  con- 
nection would  be  offered  along  the  marginal  shelf  of  a  continent 
uniting  these  now  widely  separated  countries. 


CHAPTER  XV 
THE  PRE-CAMBRIAN  SYSTEMS 

The  earth's  beginnings.  The  geological  record  does  not  tell 
us  of  the  beginnings  of  the  earth.  The  history  of  the  planet, 
as  we  have  every  reason  to  believe,  stretches  far  back  beyond 
the  period  of  the  oldest  stratified  rocks,  and  is  involved  in  the 
history  of  the  solar  system. 

The  nebular  hypothesis.  It  was  long  held  that  the  earth  began  as  a 
vaporous  shining  sphere  formed  by  the  gathering  together  of  the 
material  of  a  gaseous  ring  which  had  been  detached  from  a  cooling 
and  shrinking  nebula.  Such  a  vaporous  sphere  would  condense  to  a 
liquid  fiery  globe  whose  surface  would  become  cold  and  solid,  while 
the  interior  would  long  remain  intensely  hot  because  of  the  slow  con- 
ductivity of  the  crust.  Under  these  conditions  the  primeval  atmosphere 
of  the  earth  must  have  contained  in  vapor  the  water  now  belonging  to 
the  earth's  crust  and  surface.  It  also  held  all  the  oxygen  since  locked 
up  in  rocks  by  their  oxidation,  and  all  the  carbon  dioxide  which  has  since 
been  laid  away  in  limestones,  besides  that  corresponding  to  the  carbon 
of  carbonaceous  deposits,  such  as  peat,  coal,  and  petroleum.  On  this 
hypothesis  the  original  atmosphere  was  dense,  dark,  and  noxious,  and 
enormously  heavier  than  the  atmosphere  at  present.  Strong  objections 
have  been  raised  to  the  nebular  hypothesis,  and  it  has  been  abandoned 
by  many  authorities. 

The  planetesimal  hypothesis.  Nebulae  of  the  spheroidal  type,  such  as 
the  nebular  hypothesis  requires,  are  rare ;  but  another  type,  the  spiral, 
is  very  common.  A  spiral  nebula  consists  of  a  central  luminous  mass 
from  which  there  extend  from  opposite  sides  two  arms  more  or  less 
closely  coiled.  Each  arm  shows  luminous  knots  irregularly  placed. 

Such  a  spiral  nebula  might  be  formed  by  the  close  approach  of  one 
star  to  another,  —  of  a  passing  star  to  our  own  sun,  for  example,  before 
the  birth  of  the  solar  system.  As  the  pull  of  the  rnoon  raises  the  tides  on 
opposite  sides  of  the  earth,  so,  it  is  supposed,  the  pull  of  the  passing 

304 


THE  PRE-CAMBRIAN  SYSTEMS  305 

star  released  the  explosive  forces  of  the  sun,  and  two  streams  of  mat- 
ter were  flung  out  from  it  into  space,  one  toward  the  star  and  the  other 
directly  away  from  it.  The  knots  in  these  arms  formed  the  nuclei  of 
the  planets.  The  gaseous  matter  scattered  outside  the  knots  cooled  into 
small  stony  masses,  revolving  about  the  central  mass  and  hence  called 
planetesimals  (little  planets).  Like  the  meteorites  which  still  fall  upon 
the  earth,  the  planetesimals  were  gradually  gathered  in  by  the  nuclear 
knots,  which  thus  grew  to  the  present  planets. 

Such  cold,  stony  bodies  might  have  come  together  at  so  slow  a  rate 
that  the  heat  caused  by  their  impact  would  not  raise  sensibly  the  tem- 
perature of  the  growing  planet.  Thus  the  surface  of  the  earth  may 
never  have  been  hot  and  luminous ;  but  as  the  loose  aggregation  of 
stony  masses  grew  larger  and  was  more  and  more  compressed  by  its 
own  gravitation,  the  heat  thus  generated  raised  the  interior  to  high 
temperatures,  while  from  time  to  time  molten  rock  was  intruded  among 
the  loose,  cold  meteoritic  masses  of  the  crust  and  outpoured  upon  the 
surface. 

It  is  supposed  that  the  meteorites  of  which  the  earth  was  built 
brought  to  it,  as  meteorites  do  now,  various  gases  shut  up  within  their 
pores.  As  the  heat  of  the  interior  increased,  these  gases  transpired  to 
the  surface  and  formed  the  primitive  atmosphere  and  hydrosphere. 
The  atmosphere  has  therefore  grown  slowly  from  the  smallest  begin- 
nings. Gases  emitted  from  th'e  interior  in  volcanic  eruptions  and  in 
other  ways  have  ever  added  to  it,  and  are  adding  to  it  now.  On  the 
other  hand,  the  atmosphere  has  constantly  suffered  loss,  as  it  has  been 
robbed  of  oxygen  by  the  oxidation  of  rocks  in  weathering,  and  of 
carbon  dioxide  in  the  making  of  limestones  and  carbonaceous  deposits. 

While  all  hypotheses  of  the  earth's  beginnings  are  as  yet 
unproved  speculations,  they  serve  to  bring  to  mind  one  of  the 
chief  lessons  which  geology  has  to  teach,  —  that  the  duration  of 
the  earth  in  time,  like  the  extension  of  the  universe  in  space,  is 
vastly  beyond  the  power  of  the  human  mind  to  realize. 

We  pass  now  from  the  dim  realm  of  speculation  to  the  earli- 
est era  of  the  recorded  history  of  the  earth,  where  some  certain 
facts  may  be  observed  and  some  sure  inferences  from  them 
may  be  drawn. 


306  THE   ELEMENTS  OF  GEOLOGY 

ARCHEOZOIC  ERA  (Greek,  arc-he,  beginning;  zoe,  life): 
ARC HE AN  GROUP 

The  oldest  rocks  of  the  earth's  crust  form  an  intricate  com- 
plex of  sedimentary  and  volcanic  rocks,  crushed,  crumpled,  and 
metamorphosed,  and  injected  with  dikes,  bosses,  and  other 
igneous  intrusions.  The  base  of  the  complex  is  nowhere  to  be 
seen,  so  that  the  original  surface  of  the  earth's  crust  on  which 
the  first  sediments  were  laid  is  quite  unknown.  So  thick  are 
the  sediments  of  this  group  that  the  Archeozoic  era  may  well 
have  been  longer  than  all  later  geologic  time.  Thus,  east  of 
Lake  Huron  there  outcrop  the  upturned  edges  of  a  series  of 
metamorphosed  limestone  and  other  sedimentary  rocks,  whose 
total  thickness  is  nearly  eighteen  miles,  the  deepest  pile  of 
lime  rock  on  the  earth. 

The  well-nigh  universal  crumpling  and  metamorphism  of  the 
Archean  group,  and  the  immense  volume  of  its  igneous  intru- 
sions, prove  that  the  long  cycle  of  quiet  sedimentation  was 
closed  by  a  period  of  the  intensest  deformation.  Vast  quantities 
of  molten  rock  rose  from  the  hot  'depths  of  the  earth,  fus- 
ing the  basal  portions  of  the  Archean  sediments  and  raising 
them  into  mountains  with  granitic  cores,  while  the  strata  were 
mashed,  broken,  folded,  and  altered  on  a  scale  never  afterwards 
approached. 

The  contacts  of  the  gnarled  Archean  rocks  with  the  overlying 
sediments  show  that  before  the  opening  of  the  next  era  the 
Archeozoic  mountains  had  been  worn  down  to  their  roots. 


PROTEROZOIC  ERA:  ALGOXKIAN  GROUP 

In  some  regions  there  rests  unconformably  on  the  Archean 
an  immense  body  of  stratified  rocks,  thousands  and  in  places 
even  scores  of  thousands  of  feet  thick,  known  as  the  Algonkian. 


THE  PRE-CAMBRIAN  SYSTEMS  307 

Great  unconformities  divide  it  into  well-defined  systems,  but  as 
only  the  scantiest  traces  of  fossils  appear  here  and  there  among 
its  strata,  it  is  as  yet  impossible  to  correlate  the  formations  of 
different  regions  and  to  give  them  names  of  more  than  local 
application.  We  will  describe  the  Algonkian  rocks  of  two  typi- 
cal areas. 

The  Grand  Canyon  of  the  Colorado.  We  have  already  studied 
a  very  ancient  peneplain  whose  edge  is  exposed  to  view  deep  on 
the  walls  of  the  Colorado  Canyon  (nn1,  Fig.  207).  The  formation 
of  flat-lying  sandstone  which  covers  this  buried  land  surface  is 
proved  by  its  fossils  to  belong  to  the  Cambrian,  —  the  earliest 
period  of  the  Paleozoic  era.  The  tilted  rocks  (b,  Fig.  207)  on 
whose  upturned  edges  the  Cambrian  sandstone  rests  are  far 
older,  for  the  physical  break  which  separates  them  from  it 
records  a  tune  interval  during  which  they  were  upheaved  to 
mountainous  ridges  and  worn  down  to  a  low  plain.  They  are 
therefore  classified  as  Algonkian.  They  comprise  two  immense 
series.  The  upper  is  more  than  five  thousand  feet  thick  and 
consists  of  shales  and  sandstones  with  some  limestones.  Sepa- 
rated from  it  by  an  unconformity  which  does  not  appear  in 
Figure  207,  the  lower  division,  seven  thousand  feet  thick,  con- 
sists chiefly  of  massive  reddish  sandstones  with  seven  or  more 
sheets  of  lava  interbedded.  The  lowest  member  is  a  basal  con- 
glomerate composed  of  pebbles  derived  from  the  erosion  of  the 
dark  crumpled  schists  beneath,  —  schists  which  are  supposed  to 
be  Archean.  As  shown  in  Figure  207,  a  strong  unconformity 
(mm1,  Fig.  207)  parts  the  schists  and  the  Algonkiau.  The  floor 
on  which  the  Algonkian  rests  is  remarkably  even,  and  here 
again  is  proved  an  interval  of  incalculable  length,  during  which 
an  ancient  land  mass  of  Archean  rocks  was  baseleveled  before 
it  received  the  cover  of  the  sediments  of  the  later  age. 

The  Lake  Superior  region.  In  eastern  Canada  an  area  of 
pre-Cambrian  rocks,  Archean  and  Algonkian,  estimated  at  two 
million  square  miles,  stretches  from  the  Great  Lakes  and  the 


308 


THE  ELEMENTS  OF  GEOLOGY 


s 

I 

X 


St.  Lawrence  River  northward  to  the  confines  of 
the  continent,  inclosing  Hudson  Bay  in  the  arms 
of  a  gigantic  U.  This  immense  area,  which  we 
have  already  studied  as  the  Laurentian  peneplain 
(p.  89),  extends  southward  across  the  Canadian 
border  into  northern  Minnesota,  Wisconsin,  and 
Michigan.  The  rocks  of  this  area  are  known  to 
be  pre-Cambrian ;  for  the  Cambrian  strata,  wher- 
ever found,  lie  unconformably  upon  them. 

The  general  relations  of  the  formations  of  that 
portion  of  the  area  which  lies  about  Lake  Supe- 
rior are  shown  in  Figure  262.  Great  unconformi- 
ties, UUf,  separate  the  Algonkiaii  both  from 
the  Archean  and  from  the  Cambrian,  and  divide 
it  into  three  distinct  systems,  —  the  Lower  Hu- 
ronian, the  Upper  Huronian,  and  the  Kewee- 
nawan.  The  Lower  and  the  Upper  Huronian 
consist  in  the  main  of  old  sea  muds  and  sands 
and  limy  oozes  now  changed  to  gneisses,  schists, 
marbles,  quartzites,  slates,  and  other  metamorphic 
rocks.  The  Keweenawan  is  composed  of  immense 
piles  of  lava,  such  as  those  of  Iceland,  overlain  by 
bedded  sandstones.  What  remains  of  these  rock 
systems  after  the  denudation  of  all  later  geo- 
logic ages  is  enormous.  The  Lower  Huronian  is 
more  than  a  mile  thick,  the  Upper  Huronian  more 
than  two  miles  thick,  while  the  Keweenawan  ex- 
ceeds nine  miles  in  thickness.  The  vast  length 
of  Algonkian  time  is  shown  by  the  thickness 
of  its  marine  deposits  and  by  the  cycles  of  ero- 
sion which  it  includes.  In  Figure  262  the  stu- 
dent may  read  an  outline  of  the  history  of  the 
Lake  Superior  region,  the  deformations  which 
it-  suffered,  their  relative  severity,  the  times 


THE  PRE-CAMBRIAN  SYSTEMS  309 

when  they  occurred,  and  the  erosion  cycles  marked  by  the 
successive  unconformities. 

Other  pre-Cambrian  areas  in  North  America.  Pre-Cambrian 
rocks  are  exposed  in  various  parts  of  the  continent,  usually  by 
the  erosion  of  mountain  ranges  in  which  their  strata  were 
infolded.  Large  areas  occur  in  the  maritime  provinces  of 
Canada.  The  core  of  the  Green  Mountains  of  Vermont  is  pre- 
Cambrian,  and  rocks  of  these  systems  occur  in  scattered  patches 
in  western  Massachusetts.  Here  belong  also  the  oldest  rocks  of 
the  Highlands  of  the  Hudson  and  of  New  Jersey.  The  Adi- 
rondack region,  an  outlier  of  the  Laurentian  region,  exposes 
pre-Cambrian  rocks,  which  have  been  metamorphosed  and  tilted 
by  the  intrusion  of  a  great  boss  of  igneous  rock  out  of  which 
the  central  peaks  are  carved.  The  core  of  the  Blue  Eidge  and 
probably  much  of  the  Piedmont  Belt  are  of  this  age.  In  the 
Black  Hills  the  irruption  of  an  immense  mass  of  granite  has 
caused  or  accompanied  the  upheaval  of  pre-Cambrian  strata  and 
metamorphosed  them  by  heat  and  pressure  into  gneisses,  schists, 
quartzites,  and  slates.  In  most  of  these  mountainous  regions 
the  lowest  strata  are  profoundly  changed  by  metamorphism,  and 
they  can  be  assigned  to  the  pre-Cambrian  only  where  they  are 
clearly  overlain  unconformably  by  formations  proved  to  be  Cam- 
brian by  their  fossils.  In  the  Belt  Mountains  of  Montana,  how- 
ever, the  Cambrian  is  underlain  by  Algonkian  sediments  twelve 
thousand  feet  thick,  and  but  little  altered. 

Mineral  wealth  of  .the  pre-Cambrian  rocks.  The  pre-Cam- 
brian rocks  are  of  very  great  economic  importance,  because  of 
their  extensive  metamorphism  and  the  enormous  masses  of  igne- 
ous rock  which  they  involve.  In  many  parts  of  the  country 
they  are  the  source  of  supply  of  granite,  gneiss,  marble,  slate, 
and  other  such  building  materials.  Still  more  valuable  are  the 
stores  of  iron  and  copper  and  other  metals  which  they  contain. 

At  the  present  tune  the  pre-Cambrian  region  about  Lake 
Superior  leads  the  world  in  the  production  of  iron  ore,  its 


310         THE  ELEMENTS  OF  GEOLOGY 

output  for  1903  being  more  than  five  sevenths  of  the  entire 
output  of  the  whole  United  States,  and  exceeding  that  of  any 
foreign  country.  The  ore  bodies  consist  chiefly  of  the  red 
oxide  of  iron  (hematite)  and  occur  in  troughs  of  the  strata, 
underlain  by  some  impervious  rock.  A  theory  held  by  many 
refers  the  ultimate  source  of  the  iron  to  the  igneous  rocks  of 
the  Archean.  When  these  rocks  were  upheaved  and  subjected 
to  weathering,  their  iron  compounds  were  decomposed.  Their 
iron  was  leached  out  and  carried  away  to  be  laid  in  the  Algon- 
kian  water  bodies  in  beds  of  iron  carbonate  and  other  iron 
compounds.  During  the  later  ages,  after  the  Algonkian  strata 
had  been  uplifted  to  form  part  of  the  continent,  a  second  con- 
centration has  taken  place.  Descending  underground  waters 
charged  with  oxygen  have  decomposed  the  iron  carbonate  and 
deposited  the  iron,  in  the  form  of  iron  oxide,  in  troughs 
of  the  strata  where  their  downward  progress  was  arrested  by 
impervious  floors. 

The  pre-Cambrian  rocks  of  the  eastern  United  States  also  are 
rich  in  iron.  In  certain  districts,  as  in  the  Highlands  of  New 
Jersey,  the  black  oxide  of  iron  (magnetite)  is  so  abundant  in 
beds  and  disseminated  grains  that  the  ordinary  surveyor's  com- 
pass is  useless. 

The  pre-Cambrian  copper  mines  of  the  Lake  Superior  region 
are  among  the  richest  on  the  globe.  In  the  igneous  rocks  copper, 
next  to  iron,  is  the  most  common  of  all  the  useful  metals,  and 
it  was  especially  abundant  in  the  Keweenawan  lavas.  After 
the  Keweenawan  was  uplifted  to  form  land,  percolating  waters 
leached  out  much  of  the  copper  diffused  in  the  lava  sheets  and 
deposited  it  within  steam  blebs  as  amygdules  of  native  copper, 
in  cracks  and  fissures,  and  especially  as  a  cement,  or  matrix,  in 
the  interbedded  gravels  which  formed  the  chief  aquifers  of  the 
region.  The  famous  Calumet  and  Hecla  mine  follows  down  the 
dip  of  the  strata  to  the  depth  of  more  than  a  mile  and  works 
such  a  conglomerate  whose  matrix  is  in  part  pure  copper. 


THE  PRE-CAMBRIAN  SYSTEMS 


311 


The  appearance  of  life.  It  was  probably  during  the  Archeo- 
zoic that  life  appeared  on  earth.  Creative  energy  was  mani- 
fested upon  a  higher  plane  than  that  of  the  physical  and 
chemical  reactions  which  alone  had  made  earth's  history  so  far. 
Geology  can  offer  no  evidence  as  to  whence  or  how  life  came, 
but  it  has  much  to  reveal  of  the  marvelous  development  of 
the  forms  of  life  during  the  geologic  ages.  Its  earliest  forms 
are  unknown,  but  analogy  suggests 
that  as  every  living  creature  has 
developed  from  a  single  cell,  so  the 
earliest  organisms  upon  the  globe 

—  the  germs  from  which  all  later 
life  is  supposed  to  have  been  evolved 

—  were  tiny,  unicellular  masses  of 
protoplasm,  resembling  the  amoeba 
of  to-day  in  the  simplicity  of  their 
structure. 

Such  lowly  forms  were  destitute 
of  any  hard  parts  and  could  leave 
no  evidence  of  their  existence  in 
the  record  of  the  rocks.  And  of 
their  supposed  descendants  we  find 
so  few  traces  in  the  pre-Cambrian 
strata  that  the  first  steps  in  organic  FIG.  263.  Successive  Stages  in 
evolution  must  be  Supplied  from  the  Development  of  the  Ovum 
such  analogies  in  embryology  as  the  to  the  Gastrula  Stage 
following.  The  fertilized  ovum,  the  cell  with  which  each  ani- 
mal begins  its  life,  grows  and  multiplies  by  cell  division,  and 
develops  into  a  hollow  globe  of  cells  called  the  blastosphere. 
This  stage  is  succeeded  by  the  stage  of  the  gastrula,  —  an  ovoid 
or  cup-shaped  body  with  a  double  wall  of  cells  inclosing  a 
body  cavity,  and  with  an  opening,  the  primitive  mouth.  Each  of 
these  early  embryological  stages  is  represented  by  living  ani- 
mals, —  the  undivided  cell  by  the  protozoa,  the  blastosphere  by 


312  THE  ELEMENTS  OF  GEOLOGY 

some  rare  forms,  and  the  gastrula  in  the  essential  structure 
of  the  ccdenterates,  —  the  subkingdom  to  which  the  fresh-water 
hydra  and  the  corals  belong.  All  forms  of  animal  life,  from 
the  ccelenterates  to  the  mammals,  follow  the  same  path  in  their 
embryological  development  as  far  as  the  gastrula  stage,  but 
here  their  paths  widely  diverge,  those  of  each  subkingdom  going 
their  own  separate  ways. 

We  may  infer,  therefore,  that  during  the  pre-Cambrian  periods 
organic  evolution  followed  the  lines  thus  dimly  traced.  The 
earliest  one-celled  protozoa  were  probably  succeeded  by  many- 
celled  animals  of  the  type  of  the  blastosphere,  and  these  by 
gastrula-like  organisms.  From  the  gastrula  type  the  higher  sub- 
divisions of  animal  life  probably  diverged,  as  separate  branches 
from  a  common  trunk.  Much  or  all  of  this  vast  differentia- 
tion was  accomplished  before  the  opening  of  the  next  era ;  for 
all  the  subkingdoms  are  represented  in  the  Cambrian  except 
the  vertebrates. 

Evidences  of  pre-Cambrian  life.  An  indirect  evidence  of  life 
during  the  pre-Cambrian  periods  is  found  in  the  abundant  and 
varied  fauna  of  the  next  period ;  for,  if  the  theory  of  evolution 
is  correct,  the  differentiation  of  the  Cambrian  fauna  was  a  long 
process  which  might  well  have  required  for  its  accomplishment 
a  large  part  of  pre-Cambrian  time. 

Other  indirect  evidences  are  the  pre-Cambrian  limestones, 
iron  ores,  and  graphite  deposits,  since  such  minerals  and  rocks 
have  been  formed  in  later  times  by  the  help  of  organisms.  If  the 
carbonate  of  lime  of  the  Algonkian  limestones  and  marbles  was 
extracted  from  sea  water  by  organisms,  as  is  done  at  present  by 
corals,  mollusks,  and  other  humble  animals  and  plants,  the 
life  of  those  ancient  seas  must  have  been  abundant.  Graphite, 
a  soft  black  mineral  composed  of  carbon  and  used  in  the  man- 
ufacture of  lead  pencils  and  as  a  lubricant,  occurs  widely  in 
the  metamorphic  pre-Cambrian  rocks.  It  is  known  to  be  pro- 
duced in  some  cases  by  the  metamorphism  of  coal,  which  itself 


THE  PRE-CAMBRIAN  SYSTEMS  313 

is  formed  of  decomposed  vegetal  tissues.  Seams  of  graphite 
may  therefore  represent  accumulations  of  vegetal  matter  such 
as  seaweed.  But  limestone,  iron  ores,  and  graphite  can  be  pro- 
duced by  chemical  processes,  and  their  presence  in  the  pre- 
Cambrian  makes  it  only  probable,  and  not  certain,  that  life 
existed  at  that  time. 

Pre-Cambrian  fossils.  Very  rarely  has  any  clear  trace  of  an 
organism  been  found  in  the  most  ancient  chapters  of  the  geo- 
logical record,  so  many  of  their  leaves  have  been  destroyed  and  so 
far  have  their  pages  been  defaced.  Microscopic  radiolaria  and 
bacteria  have  been  found.  Burrows  and  trails  of  several  species 
of  annelid  worms  appear  in  the  sandstones  of  ancient  beaches. 
Fragments  have  been  reported  which  may  belong  to  brachiopods 
and  large  crustaceans.  And  in  the  limestones  of  the  Proterozoic 
occur  reefs  of  coral-like  calcareous  algae  whose  heads  are  some- 
times two  feet  thick.  These  diverse  forms  indicate  that  before 
the  Algonkian  had  closed,  life  was  abundant  and  had  widely 
differentiated.  We  may  expect  that  other  forms  will  be  dis- 
covered as  the  rocks  are  closely  searched. 

Pre-Cambrian  geography.  Our  knowledge  is  far  too  meager 
to  warrant  an  attempt  to  draw  the  varying  outlines  of  sea  and 
land  during  the  Archean  and  Algonkian  eras.  Pre-Cambrian 
time  probably  was  longer  than  all  later  geological  time  down 
to  the  present,  as  we  may  infer  from  the  vast  thicknesses  of  its 
rocks  and  the  unconformities  which  part  them.  We  know  that 
during  its  long  periods  land  masses  again  and  again  rose  from 
the  sea,  were  worn  low,  and  were  submerged  and  covered  with 
the  waste  of  other  lands.  But  the  formations  of  separated 
regions  cannot  be  correlated  because  of  the  absence  of  fossils, 
and  nothing  more  can  be  made  out  than  the  detached  chapters 
of  local  histories,  such  as  the  outline  given  of  the  district  about 
Lake  Superior. 

The  pre-Cambrian  rocks  show  no  evidence  of  any  forces  then 
at  work  upon  the  earth  except  the  forces  which  are  at  work 


314  THE  ELEMENTS  OF  GEOLOGY 

upon  it  now.  The  most  ancient  sediments  known  are  so  like 
the  sediments  now  being  laid  that  we  may  infer  that  they  were 
formed  under  conditions  essentially  similar  to  those  of  the 
present  time.  There  is  no  proof  that  the  sands  of  the  pre- 
Cambrian  sandstones  were  swept  by  any  more  powerful  waves 
and  currents  than  are  offshore  sands  to-day,  or  that  the  muds 
of  the  pre-Cambrian  shales  settled  to  the  sea  floor  in  less  quiet 
water  than  such  muds  settle  in  at  present.  The  pre-Cambrian 
lands  were,  no  doubt,  worn  by  wind  and  weather,  beaten  by 
rain,  and  furrowed  by  streams  as  now,  and,  as  now,  they 
fronted  the  ocean  with  beaches  on  which  waves  dashed  and 
along  which  tidal  currents  ran.  They  wrere  also  scoured  by 
continental  ice  sheets  during  recurring  glacial  epochs,  for  amid 
the  Proterozoic  rocks  widespread  ground  moraines  are  found 
in  Canada,  India,  South  Africa,  and  Australia. 

Perhaps  the  chief  difference  between  the  pre-Cambrian  and 
the  present  was  the  absence  of  life  upon  the  land.  So  far  as  we 
have  any  knowledge,  no  forests  covered  the  mountain  sides,  no 
verdure  carpeted  the  plains,  and  no  animals  lived  on  the 
ground  or  in  the  air.  It  is  permitted  to  think  of  the  most 
ancient  lands  as  deserts  of  barren  rock  and  rock  waste  swept 
by  rains  and  trenched  by  powerful  streams.  We  may  there- 
fore suppose  that  the  processes  of  their  destruction  went  on 
more  rapidly  than  at  present. 


CHAPTEE  XVI 
THE  CAMBRIAN 

The  Paleozoic  era.  The  second  volume  of  the  geological 
record,  called  the  Paleozoic  (Greek,  palaios,  ancient ;  zoe,  life), 
has  come  down  to  us  far  less  mutilated  and  defaced  than  has 
the  first  volume,  which  contains  the  traces  of  the  most  ancient 
life  of  the  globe.  Fossils  are  far  more  abundant  in  the  Paleo- 
zoic than  in  the  earlier  strata,  while  the  sediments  in  which 
they  were  entombed  have  suffered  far  less  from  metamorphism 
and  other  causes,  and  have  been  less  widely  buried  from  view, 
than  the  strata  of  the  pre-Cambrian  groups.  By  means  of  their 
fossils  we  can  correlate  the  formations  of  widely  separated 
regions  from  the  beginning  of  the  Paleozoic  on,  and  can  there- 
fore trace  some  outline  of  the  history  of  the  continents. 

Paleozoic  time,  although  shorter  than  the  pre-Cambrian  as 
measured  by  the  thickness  of  the  strata,  must  still  be  reckoned 
in  millions  of  years.  During  this  vast  reach  of  time  the  changes 
in  organisms  were  very  great.  It  is  according  to  the  successive 
stages  in  the  advance  of  life  that  the  Paleozoic  formations  are 
arranged  in  five  systems,  —  the  Cambrian,  the  Ordovician,  the 
Silurian,  the  Devonian,  and  the  Carboniferous.  On  the  same 
basis  the  first  three  systems  are  grouped  together  as  the  older 
Paleozoic,  because  they  alike  are  characterized  by  the  dominance 
of  the  invertebrates ;  while  the  last  two  systems  are  united  in 
the  later  Paleozoic,  and  are  characterized,  the  one  by  the  domi- 
nance of  fishes,  and  the  other  by  the  appearance  of  amphibians 
and  reptiles. 

Each  of  these  systems  is  world-wide  in  its  distribution,  and 
may  be  recognized  on  any  continent  by  its  own  peculiar  fauna. 

315 


316          THE  ELEMENTS  OF  GEOLOGY 

The  names  first  given  them  in  Great  Britain  have  therefore 
come  into  general  use,  while  their  subdivisions,  which  often 
cannot  be  correlated  in  different  countries  and  different  regions, 
are  usually  given  local  names. 

The  first  three  systems  were  named  from  the  fact  that  their  strata 
are  well  displayed  in  Wales.  The  Cambrian  carries  the  Roman  name 
of  Wales,  and  the  Ordovician  and  Silurian  the  names  of  tribes  of 
ancient  Britons  which  inhabited  the  same  country.  The  Devonian  is 
named  from  the  English  county  Devon,  where  its  rocks  were  early 
studied.  The  Carboniferous  was  so  called  from  the  large  amount  of 
coal  which  it  was  found  to  contain  in  Great  Britain  and  continental 
Europe. 

THE  CAMBRIAN 

Distribution  of  strata.  The  Cambrian  rocks  outcrop  in  narrow 
belts  about  the  pre-Cambrian  areas  of  eastern  Canada  and  the 
Lake  Superior  region,  the  Adirondacks  and  the  Green  Mountains. 
Strips  of  Cambrian  formations  occupy  troughs  in  the  pre-Cam- 
brian rocks  of  New  England  and  the  maritime  provinces  of 
Canada ;  a  long  belt  borders  on  the  west  the  crystalline  rocks  of 
the  Blue  Ridge ;  and  on  the  opposite  side  of  the  continent  the 
Cambrian  reappears  in  the  mountains  of  the  Great  Basin  and  the 
Canadian  Rockies.  In  the  Mississippi  valley  it  is  exposed  in 
small  districts  where  uplift  has  permitted  the  stripping  off'  of 
younger  rocks.  Although  the  areas  of  outcrop  are  small,  we 
may  infer  that  Cambrian  rocks  were  widely  deposited  over  the 
continent  of  North  America. 

Physical  geography.  The  Cambrian  system  of  North  Amer- 
ica comprises  three  distinct  series,  the  Lower  Cambrian,  the 
Middle  Cambrian,  and  the  Upper  Cambrian,  each  of  which  is 
characterized  by  its  own  peculiar  fauna.  In  sketching  the  out- 
lines of  the  continent  as  it  was  at  the  beginning  of  the  Paleozoic, 
it  must  be  remembered  that  wherever  the  Lower  Cambrian 
formations  now  are  found  was  certainly  then  sea  bottom,  and 


THE  CAMBRIAN 


317 


wherever  the.  Lower  Cambrian  are  wanting,  and  the  next  forma- 
tions rest  directly  on  pre-Cambrian  rocks,  was  probably  then  land. 

Early  Cambrian  geography.  In  this  way  we  know  that  at 
the  opening  of  the  Cambrian  two  long,  narrow  mediterranean 
seas  stretched  from  north  to  south  across  the  continent.  The 
eastern  sea  extended  from  the  Gulf  of  St.  Lawrence  down  the 
Champlain-Hudson 
valley  and  thence 
along  the  western 
base  of  the  Blue 
Eidge  south  at 
least  to  Alabama. 
The  western  sea 
stretched  from  the 
Canadian  Rockies 
over  the  Great 
Basin  and  at  least 
as  far  south  as  the 
Grand  Canyon  of 
the  Colorado  in 
Arizona. 

Between  these 
mediterraneans  lay 
a  great  central  land 
which  included  the 
pre-Cambrian  U- 
shaped  area  of  the 
Laurentian  peneplain,  and  probably  extended  southward  to  the 
latitude  of  New  Orleans.  To  the  east  lay  a  land  which  we  may 
designate  as  Appalachia,  whose  western  shore  line  was  drawn 
along  the  site  of  the  present  Blue  Ridge,  but  whose  other  limits 
are  quite  unknown.  The  land  of  Appalachia  must  have  been 
large,  for  it  furnished  a  great  amount  of  waste  during  the  entire 
Paleozoic  era,  and  its  eastern  coast  may  possibly  have  lain  even 


FIG.  264.    Hypothetical   Map   of  Eastern   North 
America  at  the  Beginning  of  Cambrian  Time 

Unshaded  areas,  probable  land 


318         THE  ELEMENTS  OF  GEOLOGY 

beyond  the  edge  of  the  present  continental  shelf.  On  the  west- 
ern side  of  the  continent  a  narrow  land  occupied  the  site  of  the 
Sierra  Nevada  Mountains. 

Thus,  even  at  the  beginning  of  the  Paleozoic,  the  continental 
plateau  of  North  America  had  already  been  left  by  crustal  move- 
ments in  relief  above  the  abysses  of  the  great  oceans  on  either 
side.  The  mediterraneans  which  lay  upon  it  were  shallow,  as 
their  sediments  prove.  They  were  epicontinental  seas;  that  is, 
they  rested  upon  (Greek,  epi)  the  submerged  portion  of  the  con- 
tinental plateau.  We  have  no  proof  that  the  deep  ocean  ever 
occupied  any  part  of  where  North  America  now  is. 

The  Middle  and  Upper  Cambrian  strata  are  found  together 
with  the  Lower  Cambrian  over  the  area  of  both  the  eastern 
and  the  western  mediterraneans,  so  that  here  the  sea  contin- 
ued during  the  entire  period.  The  sediments  throughout  are 
those  of  shoal  water.  Coarse  cross-bedded  sandstones  record 
the  action  of  strong  shifting  currents  which  spread  coarse  waste 
near  shore  and  winnowed  it  of  finer  stuff.  Frequent  ripple 
marks  on  the  bedding  planes  of  the  strata  prove  that  the  loose 
sands  of  the  sea  floor  were  near  enough  to  the  surface  to  be 
agitated  by  waves  and  tidal  currents.  Sun  cracks  show  that 
often  the  outgoing  tide  exposed  large  muddy  flats  to  the  drying 
action  of  the  sun.  The  fossils,  also,  of  the  strata  are  of  kinds 
related  to  those  which  now  live  in  shallow  waters  near  the 
shore. 

The  sediments  which  gathered  in  the  mediterranean  seas 
were  very  thick,  reaching  in  places  the  enormous  depth  of  ten 
thousand  feet.  Hence  the  bottoms  of  these  seas  were  sinking 
troughs,  ever  filling  with  waste  from  the  adjacent  land  as  fast 
as  they  subsided. 

Late  Cambrian  geography.  The  formations  of  the  Middle 
and  Upper  Cambrian  are  found  resting  unconformably  on  the 
pre-Cambrian  rocks  from  New  York  westward  into  Minnesota 
and  at  various  points  in  the  interior,  as  in  Missouri  and  in 


THE  CAMBRIAN  319 

Texas.  Hence  after  earlier  Cambrian  time  the  central  land  sub- 
sided, with  much  the  same  effect  as  if  the  Mississippi  valley 
were  now  to  lower  gradually,  and  the  Gulf  of  Mexico  to  spread 
northward  until  it  entered  Lake  Superior.  The  Cambrian  seas 
transgressed  the  central  land  and  strewed  far  and  wide  behind 
their  advancing  beaches  the  sediments  of  the  later  Cambrian 
upon  an  eroded  surface  of  pre-Cambrian  rocks. 

The  succession  of  the  Cambrian  formations  in  North  America 
records  many  minor  oscillations  and  varying  conditions  of 
physical  geography ;  yet  on  the  whole  it  tells  of  widening  seas 
and  lowering  lands.  Basal  conglomerates  and  coarse  sandstones 
which  must  have  been  laid  near  shore  are  succeeded  by  shaly 
sandstones,  sandy  shales,  and  shales.  Toward  the  top  of  the 
series  heavy  beds  of  limestone,  extending  from  the  Blue  Ridge 
to  Missouri,  speak  of  clear  water,  and  either  of  more  distant 
shores  or  of  neighboring  lands  which  were  worn  or  sunk  so 
low  that  for  the  most  part  their  waste  was  carried  to  the  sea 
in  solution. 

In  brief,  the  Cambrian  was  a  period  of  submergence.  It 
began  with  the  larger  part  of  North  America  emerged  as  great 
land  masses.  It  closed  with  most  of  the  interior  of  the  con- 
tinental plateau  covered  with  a  shallow  sea. 

THE  LIFE  OF  THE  CAMBRIAN  PERIOD 

It  is  now  for  the  first  tune  that  we  find  preserved  in  the 
offshore  deposits  of  the  Cambrian  seas  enough  remains  of  ani- 
mal life  to  be  properly  called  a  fauna.  Doubtless  these  remains 
are  only  the  most  fragmentary  representation  of  the  life  of  the 
time,  for  the  Cambrian  rocks  are  very  old  and  have  been  widely 
metamorphosed.  Yet  the  five  hundred  and  more  species  already 
discovered  embrace  all  the  leading  types  of  invertebrate  life,  and 
are  so  varied  that  we  must  believe  that  their  lines  of  descent 
stretch  far  back  into  the  pre-Cambrian  past. 


320 


THE  ELEMENTS  OF  GEOLOGY 


Plants.    No  remains  of  plants  have  been  found  in  Cambrian 
strata,  except  some  doubtful  markings,  as  of  seaweed. 

Sponges.  The  sponges,  the  lowest 
of  the  multicellular  animals,  were 
represented  by  several  orders.  Their 
fossils  are  recognized  by  the  sili- 
ceous spicules,  which,  as  in  modern 
sponges,  either  were  scattered 
through  a  mass  of  horny  fibers  or 
were  connected  in  a  flinty  frame- 
work. 

F.o.266.  sponge  Spicules  as  Coelenterates.  This  subkingdom 
seen  in  Flint  under  the  includes  two  classes  of  interest  to 
Microscope  the  geologist,  —  the  Hydrozoa,  such 

as  the  fresh-water  hydra  and  the  jellyfish,  and  the  corals.    Both 
classes  existed  in  the  Cambrian. 

The  Hydrozoa  were  represented  not   only  by  jellyfish  but 
also  by  the  graptolite,  which  takes  its   name  from  a  fancied 


FIG.  200.    Graptolites,  Ordovician  and  Silurian  Species 


THE  CAMBRIAN 


321 


resemblance  of 'some  of  its  forms  to  a  quill  pen.  It  was  a  com- 
posite animal  with  a  horny  framework,  the  individuals  of  the 
colony  living  in  cells  strung  on  one  or  both  sides  along  a  hollow 
stem,  and  communicating  by  means  of  a  common  flesh  in  this 
central  tube.  Some  graptolites  were  straight,  and  some  curved 
or  spiral;  some  were  single  stemmed,  and  others  consisted  of 
several  radial  stems  united.  Graptolites  occur  but  rarely  in 
the  Upper  Cambrian.  In  the  Ordovician  and  Silurian  they  are 
very  plentiful,  and  at 
the  close  of  the  Silurian 
they  pass  out  of  exist- 
ence, never  to  return. 

Corals  are  very  rarely 
found  in  the  Cambrian, 
and  the  description  of 
their  primitive  types  is 
postponed  to  later  chap- 
ters treating  of  periods 
when  they  became  more 
numerous. 

Echinoderms.  This 
subkingdom  comprises 
at  present  such  familiar 
forms  as  the  crinoid,  the 
starfish,  and  the  sea  urchin.  The  structure  of  echinoderms  is 
radiate.  Their  integument  is  hardened  with  plates  or  particles 
of  carbonate  of  lime. 

Of  the  free  echinoderms,  such  as  the  starfish  and  the  sea  urchin, 
the  former  'has  been  found  in  the  Cambrian  rocks  of  Europe, 
but  neither  have  so  far  been  discovered  in  the  strata  of  this 
period  in  North  America,  The  stemmed  and  lower  division  of 
the  echinoderms  was  represented  by  a  primitive  type,  the 
cystoid,  so  called  from  its  saclike  form.  A  small  globular  or 
ovate  "calyx"  of  calcareous  plates,  with  an  aperture  at  the 


FIG.  267.   Cystoids,  one  showing  Two  Rudi- 
mentary Arms 


322 


THE  ELEMENTS  OF  GEOLOGY 


top  for  the  mouth,  inclosed  the  body  of  the  animal,  and  was 
attached  to  the  sea  bottom  by  a  short  flexible  stalk  consisting 
of  disks  of  carbonate  of  lime  held  together  by  a  central  ligament. 
Arthropods.  These  segmented  animals  with  "  jointed  feet," 
as  their  name  suggests,  may  be  divided  in  a  general  way  into 
water  breathers  and  air  breathers.  The  first-named  and  lower 
division  comprises  the  class  of  the  Crustacea,  —  arthropods 
protected  by  a  hard  exterior  skeleton,  or  "  crust,"  —  of  which 
crabs,  crayfish,  and  lobsters  are  familiar  examples.  The  higher 


FIG.  268.   Trilobites 

A,  a  Cambrian  species;  B,  a  Devonian  species,  showing  eye;  C,  restoration 
of  an  Ordovician  species 

division,  that  of  the  air  breathers,  includes  the  following  classes  : 
spiders,  scorpions,  centipedes,  and  insects. 

The  trilobite.  The  aquatic  arthropods,  the  Crustacea,  culmi- 
nated before  the  air  breathers ;  and  while  none  of  the  latter  are 
found  in  the  Cambrian,  the  former  were  the  dominant  life  of 
the  time  in  numbers,  in  size,  and  in  the  variety  of  their  forms. 
The  leading  crustacean  type  is  the  trilobite,  which  takes  its 
name  from  the  three  lobes  into  which  its  shell  is  divided  longi- 
tudinally. There  are  also  three  cross  divisions, — the  head  shield, 


THE   CAMBRIAN  323 

the  tail  shield, 'and  between  the  two  the  thorax,  consisting  of  a 
number  of  distinct  and  unconsolidated  segments.  The  head 
shield  carries  a  pair  of  large,  crescentic,  compound  eyes,  like 
those  of  the  insect.  The  eye  varies  greatly  in  the  number  of  its 
lenses,  ranging  from  fourteen  in  some  species  to  fifteen  thousand 
in  others.  Figure  268,  (7,  is  a  restoration  of  the  trilobite,  and 
shows  the  appendages,  which  are  found  preserved  only  in  the 
rarest  cases. 

During  the  long  ages  of  the  Cambrian  the  trilobite  varied 
greatly.  Again  and  again  new  species  and  genera  appeared, 
while  the  older  types  became  extinct.  For  this  reason  and 
because  of  their  abundance,  trilobites  are  used  in  the  classifica- 
tion of  the  Cambrian  system.  The  Lower  Cambrian  is  charac- 
terized by  the  presence  of  a  trilobitic  fauna  in  which  the  genus 
Olenellus  is  predominant.  This,  the  Ole- 
nellus  Zone,  is  one  of  the  most  important 
platforms  in  the  entire  geological  series  ; 
for,  the  wTorld  over,  it  marks  the  begin- 
ning of  Paleozoic  time,  while  all  under- 
lying strata  are  classified  as  pre-Cam- 

.     ,        FIG.  269.  APhyllopod 
brian.    The  Middle  Cambrian  is  marked 

by  the  genus  Paradoxides,  and  the  Upper  Cambrian  by  the 
genus  Olenus.  Some  of  the  Cambrian  trilobites  were  giants, 
measuring  as  much  as  two  feet  long,  while  others  were  the 
smallest  of  their  kind,  a  fraction  of  an  inch  in  length. 

Another  type  of  crustacean  which  lived  in  the  Cambrian  and 
whose  order  is  still  living  is  illustrated  in  Figure  269. 

Worms.  Trails  and  burrows  of  worms  have  been  left  on 
the  sea  beaches  and  mud  flats  of  all  geological  times  from  the 
Algonkian  to  the  present. 

Brachiopods.  These  soft-bodied  animals,  with  bivalve  shells 
and  two  interior  armlike  processes  which  served  for  breathing, 
appeared  in  the  Algonkian,  and  had  now  become  very  abundant. 
The  two  valves  of  the  brachiopod  shell  are  unequal  in  size,  and 


324 


THE   ELEMENTS  OF   GEOLOGY 


in  each  valve  a  line  drawn  from  the  beak  to  the  base  divides 
the  valve  into  two  equal  parts  (Fig.  270).    It  may  thus  be  told 

from  the  pelecypod  mollusk,  such  as 
the  clam,  whose  two  valves  are  not  far 
from  equal  in  size,  each  being  divided 
into  unequal  parts  by  a  line  dropped 
from  the  beak  (Fig.  272). 
FIG.  270.  A  Cambrian  Articu-        Brachiopods  include  two  orders.   In 
late  Brachiopod,  Orthis        the  mogt  primitive  order  —  that  of  the 

inarticulate  brachiopods  —  the   two  valves   are  held   together 

only  by  muscles  of  the  animal,  and  the  shell  is  horny  or  is 

composed  of  phosphate  of  lime.     The  Discina,  which  began  in 

the  Algonkian,  is  of  this  type,  as  is 

also  the  Lingulella  of  the  Cambrian 

(Fig.  271).     Both    of   these  genera 

have  lived  on  during  the  millions 

of   years  of   geological   time   since 

their    introduction,    handing    down    FIG.  271.   Cambrian 


late  Brachiopods 
A,  Lingulella ;  B,  Discina 


from  generation  to  generation  with 
hardly  any  change  to  their  descend- 
ants now  living  off  our  shores  the  characters  impressed  upon 
them  at  the  beginning. 

The  more  highly  organized  articulate  brachiopods  have 
valves  of  carbonate  of  lime  more  securely  joined  by  a  hinge 
with  teeth  and  sockets  (Fig.  270).  In  the 
Cambrian  the  inarticulates  predominate, 
though  the  articulates  grow  common 
toward  the  end  of  the  period.. 

Mollusks.  The  three  chief  classes  of 
mollusks  — the  pelecypods  (represented  by 
the  oyster  and  clam  of  to-day),  the  gastro- 
pods (represented  now  by  snails,  conches,  and  periwinkles),  and 
the  ceplialopods  (such  as  the  nautilus,  cuttlefish,  and  squids)  — 
were  all  represented  in  the  Cambrian,  although  very  sparingly. 


FiG.  272.   A  Cambrian 
Pelecypod 


THE   CAMBRIAN 


325 


Pteropods,  a  suborder  of  the  gastropods,  appeared  in  this  age. 
Their  papery  shells  of  carbonate  of  lime  are  found  in  great  num- 
bers from  this  time  on. 


FIG.  273.    Gastropods,  Ordovician  Species 

Cephalopods,  the  most  highly  organized  of  the  mollusks,  started 
into  existence,  so  far  as  the  record  shows,  toward  the  end  of 
the  Cambrian,  with  the 
long  extinct  OrtJioceras 
(straigJithorri)  and  the 
allied  genera  of  its 
family.  The  Orthoceras 
had  a  long,  straight,  and 
tapering  shell,  divided 
by  cross  partitions  into 
chambers.  The  animal 
lived  in  the  "body 
chamber"  at  the  larger 
end,  and  walled  off  the 
other  chambers  from  it 
in  succession  during  the  growth  of  the  shell  A  central  tube. 


FIG.  274.   Cambrian  Pteropods 


326 


THE  ELEMENTS  OF  GEOLOGY 


the  siplmncle  (s,  Fig.  275,  B\  passed  through  from  the  body 

chamber  to  the  closed  tip  of  the  cone. 

The  seashells,  both  brachiopods 
and  mollusks,  are  in  some  respects 
the  most  important  to  the  geologist 
of  all  fossils.  They  have  been  so 
numerous,  so  widely  distributed, 
and  so  well  preserved  because  of 
their  durable  shells  and  their 
station  in  growing  sediments,  that 
better  than  any  other  group  of 
organisms  they  can  be  used  to  cor- 
relate the  strata  of  different  regions 
and  to  mark  by  their  slow  changes 
the  advance  of  geological  time. 

Climate.  The  life  of  Cambrian 
times  in  different  countries  con- 
tains no  suggestion  of  any  marked 

climatic  zones,  and  as  in  later  periods  a  warm  climate  probably 

reached  to  the  polar  regions. 


B 

FIG.  275.    Orthoceras 
A,  fossil  shell;  B,  restoration 


CHAPTER  XVII 

THE  ORDOVICIAN  *  AND  SILURIAN 

THE  ORDOVICIAN 

In  North  America  the  Ordovician  rocks  lie  conformably  on 
the  Cambrian.  The  two  periods,  therefore,  were  not  parted  by 
any  deformation,  either  of  mountain  making  or  of  continental 
uplift.  The  general  submergence  which  marked  the  Cambrian 
continued  into  the  succeeding  period  with  little  interruption. 

Subdivisions  and  distribution  of  strata.  The  Ordovician 
series,  as  they  have  been  made  out  in  New  York,  are  given  for 
reference  in  the  following  table,  with  the  rocks  of  which  they 
are  chiefly  composed : 

5  Hudson shales 

4  Utica shales 

3  Trenton limestones 

2  Chazy limestones 

1  Calciferous sandy  limestones 

These  marine  formations  of  the  Ordovician  outcrop  about  the 
Cambrian  and  pre-Cambrian  areas,  and,  as  borings  show,  extend 
far  and  wide  over  the  interior  of  the  continent  beneath  more 
recent  strata.  The  Ordovician  sea  stretched  from  Appalachia 
across  the  Mississippi  valley.  It  seems  to  have  extended  to 
California,  although  broken  probably  by  several  mountainous 
islands  in  the  west. 

Physical  geography.  The  physical  history  of  the  period  is 
recorded  in  the  succession  of  its  formations.  The  sandstones  of 
the  Upper  Cambrian,  as  we  have  learned,  tell  of  a  transgressing 

i  Often  known  as  the  Lower  Silurian. 
327 


328 


THE  ELEMENTS  OF  GEOLOGY 


sea  which  gradually  came  to  occupy  the  Mississippi  valley  and 
the  interior  of  North  America.  The  limestones  of  the  early  and 
middle  Ordovician  show  that  now  the  shore  had  become  remote 
and  the  lands  had  become  more  low.  The  waters  now  had 
cleared.  Colonies  of  brachiopods  and  other  lime-secreting  ani- 
mals occupied  the  sea  bottom,  and  their  debris  mantled  it  with 
sheets  of  limy  ooze.  The  sandy  limestones  of  the  Calciferous 

record  the  transition 
stage  from  the  Cam- 
brian when  some  sand 
was  still  brought  in 
from  shore.  The  highly 
fossilif erous  limestones 
of  the  Trenton  tell  of 
clear  water  and  abun- 
dant life.  We  need  not 
regard  this  epiconti- 
nental  sea  as  deep.  No 
abysmal  deposits  have 
been  found,  and  the 
limestones  of  the  period 
are  those  which  would 
be  laid  in  clear,  warm 
water  of  moderate 
depth,  like  that  of 
modern  coral  seas. 
The  shales  of  the  Utica  and  Hudson  show  that  the  waters  of 
the  sea  now  became  clouded  with  mud  washed  in  from  land. 
Either  the  land  was  gradually  uplifted,  or  perhaps  there  had 
arrived  one  of  those  periodic  crises  which,  as  we  may  imagine, 
have  taken  place  whenever  the  crust  of  the  shrinking  earth  has 
slowly  given  way  over  its  great  depressions,  and  the  ocean  has 
withdrawn  its  waters  into  deepening  abysses.  The  land  was 
thus  left  relatively  higher  and  bordered  with  new  coastal  plains. 


FIG.  276.   Hypothetical  Map  of  the  Eastern 
United  States  in  Ordovician  Time 

Shaded  areas,  probable  sea;  broken  lines,  ap- 
proximate shore  lines 


THE  ORDOVICIAN  AND  SILURIAN  329 

The  epicontinental  sea  was  shoaled  and  narrowed,  and  muds 
were  washed  in  from  the  adjacent  lands. 

The  Taconic  deformation.  The  Ordovician  was  closed  by  a 
deformation  whose  extent  and  severity  are  not  yet  known. 
From  the  St.  Lawrence  Eiver  to  New  York  Bay,  along  the 
northwestern  and  western  border  of  New  England,  lies  a  belt  of 
Cambrian-Ordovician  rocks  more  than  a  mile  in  total  thickness, 
which  accumulated  during  the  long  ages  of  those  periods  in  a 
gradually  subsiding  trough  between  the  Adirondacks  and  a  pre- 
Carnbrian  range  lying  west  of  the  Connecticut  River.  But  since 
their  deposition  these  ancient  sediments  have  been  crumpled 
and  crushed,  broken  with  great  faults,  and  extensively  metamor- 
phosed. The  limestones  have  recrystallized  into  marbles,  among 
them  the  famous  marbles  of  Vermont ;  the  Cambrian  sandstones 
have  become  quartzites,  and  the  Hudson  shale  has  been  changed 
to  a  schist  exposed  on  Manhattan  Island  and  northward. 

In  part  these  changes  occurred  at  the  close  of  the  Ordovician, 
for  in  several  places  beds  of  Silurian  age  rest  unconformably  on 
the  upturned  Ordovician  strata ;  but  recent  investigations  have 
made  it  probable  that  the  crustal  movements  recurred  at  later 
times,  and  it  was  perhaps  in  the  Devonian  and  at  the  close  of 
the  Carboniferous  that  the  greater  part  of  the  deformation  and 
metamorphism  was  accomplished.  As  a  result  of  these  move- 
ments,—  perhaps  several  times  repeated,  —  a  great  mountain 
range  was  upridged,  which  has  been  long  since  leveled  by  ero- 
sion, but  whose  roots  are  now  visible  in  the  Taconic  Mountains 
of  western  New  England. 

The  Cincinnati  anticline.  Over  an  oval  area  in  Ohio,  Indiana,  and 
Kentucky,  whose  longer  axis  extends  from  north  to  south  through 
Cincinnati,  the  Ordovician  strata  rise  in  a  very  low,  broad  swell,  called 
the  Cincinnati  anticline.  The  Silurian  and  Devonian  strata  thin  out  as 
they  approach  this  area  and  seem  never  to  have  deposited  upon  it.  We 
may  regard  it,  therefore,  as  an  island  upwarped  from  the  sea  at  the 
close  of  the  Ordovician  or  shortly  after. 


330 


THE  ELEMENTS  OF  GEOLOGY 


Petroleum  and  natural  gas.  These  valuable  illuminants  and 
fuels  are  considered  here  because,  although  they  are  found  in 
traces  in  older  strata,  it  is  in  the  Ordovician  that  they  occur 
for  the  first  time  in  large  quantities.  They  range  throughout 
later  formations  down  to  the  most  recent. 

The  oil  horizons  of  California  and  Texas  are  Tertiary ;  those  of  Col- 
orado, Cretaceous;  those  of  West  Virginia,  Carboniferous;  those  of 
Pennsylvania,  Kentucky,  and  Canada,  Devonian ;  and  the  large  field 
of  Ohio  and  Indiana  belongs  to  the  Ordovician  and  higher  systems. 

Petroleum  and  natural  gas,  wherever  found,  have  probably 
originated  from  the  decay  of  organic  matter  when  buried  in 
sedimentary  deposits,  just  as  at  present  in  swampy  places  the 
hydrogen  and  carbon  of  decaying  vegetation  combine  to  form 
marsh  gas.  The  light  and  heat  of  these  hydrocarbons  we  may 
think  of,  therefore,  as  a  gift  to  the  civilized  life  of  our  race  from 
d  d'  d"  the  humble  organisms,  both 

animal  and  vegetable,  of  the 
remote  past,  whose  remains 
were  entombed  in  the  sedi- 
ments of  the  Ordovician  and 
later  geological  ages. 
FIG.  277.    Diagram  illustrating  the  Con-         Petroleum  is  very  widely 
ditions  of  Accumulation  of  Oil  and  disseminated    throughout 

the  stratified  rocks.    Certain 

swun^,    b,  reservoir;    c,  cover.    What    ,.  --11 

would  be  the  result  of  boring  to  the  limestones  are  visibly  greasy 

reservoir  rock  at  d?  at  d'?  at  d"?  witri  it,   and  others  give  off 

its  characteristic  fetid  odor  when  struck  with  a  hammer.  Many 
shales  are  bituminous,  and  some  are  so  highly  charged  that 
small  flakes  may  be  lighted  like  tapers,  and  several  gallons  of 
oil  to  the  ton  may  be  obtained  by  distillation. 

But  oil  and  gas  are  found  in  paying  quantities  only  when  cer- 
tain conditions  meet : 

1.  A  source  below,  usually  a  bituminous  shale,  from  whose 
organic  matter  they  have  been  derived  by  slow  change. 


Gas 
a,  source 


THE  ORDOVICIAN  AND  SILURIAN  331 

2.  A  reservoir  above,  in  which  they  have  gathered.    This  is 
either  a  porous  sandstone  or  a  porous  or  creviced  limestone. 

3.  Oil  and  gas  are  lighter  than  water,  and  are  usually  under 
pressure  owing  to  artesian  water.    Hence,  in  order  to  hold  them 
from  escaping  to  the  surface,  the  reservoir  must  have  the  shape 
of  an  anticline,  dome,  or  lens. 

4.  It  must  also  have  an  impervious  cover,  usually  a  shale. 
In  these  reservoirs  gas  is  under  a  pressure  which  is  often 

enormous,  reaching  in  extreme  cases  as  high  as  a  thousand  five 
hundred  pounds  to  the  square  inch.  When  tapped  it  rushes 
out  with  a  deafening  roar,  sometimes  flinging  the  heavy  drill 
high  in  air.  In  accounting  for  this  pressure  we  must  remember 
that  the  gas  has  been  compressed  within  the  pores  of  the  reser- 
voir rock  by  artesian  water,  and  in  some  cases  also  by  its  own 
expansive  force.  It  is  not  uncommon  for  artesian  water  to  rise 
in  wells  after  the  exhaustion  of  gas  and  oil 

Life  of  the  Ordovician 

During  the  ages  of  the  Ordovician,  life  made  great  advances. 
Types  already  present  branched  widely  into  new  genera  and 
species,  and  new  and  higher 
types  appeared. 

Sponges  continued  from 
the  Cambrian.  Graptolites 
now  reached  their  climax. 

Stromatopora  —  colonies 
of  minute  hydrozoans  allied 
to  corals  —  grew  in  places 

on  the  sea  floor,  secreting 

FIG.  278.   Stromatopora 
stony  masses  composed  of 

thin,  close,  concentric  layers,  connected  by  vertical  rods.  The 
Stromatopora  are  among  the  chief  limestone  builders  of  the 
Silurian  and  Devonian  periods. 


332 


THE  ELEMENTS  OF  GEOLOGY 


Corals  developed  along  several  distinct  lines, 
like  modern  corals  they  secreted  a  calcareous 
framework,  in  whose  outer  portions  the  polyps 
lived.  In  the  Ordovician,  corals  were  represented 
chiefly  by  the  family  of  the  Chcetetes,  all  species 
of  which  are  long  since  extinct.  The  description 
of  other  types  of  corals  will  be  given  under  the 
Silurian,  where  they  first  became  abundant. 

Echinoderms.  The  cystoid  reaches  its  climax,  but 
there  appear  now  two  higher  types  of  echinoderms, 
—  the  crinoid  and  the  starfish.  The  crinoid,  named 
from  its  resemblance  to  the  lily,  is  like  the  cystoid 
in  many  respects,  but  has  a  longer  stem  and  sup- 
ports a  crown  of  plumose  arms.  Stirring  the  water 
with  these  arms,  it  creates  currents  by  which  par- 
ticles of  food  are  wafted  to  its  mouth.  Crinoids  are 
rare  at  the  present  time,  but  they  grew  in  the 
greatest  profusion  in  the  warm  Ordovician  seas  and 
for  long  ages  thereafter.  In  many  places  the  sea 
floor  was  beautiful  with  these  graceful,  flowerlike 
forms,  as  with  fields  of  long-stemmed  lilies.  Of  the 
higher,  free-moving  classes  of  the  echinoderms,  star- 
fish are  more  numerous  than  in  the  Cambrian,  and 
sea  urchins  make  their  appearance  in  rare  ar- 
chaic forms. 

Crustaceans.   Trilobites  now  reach  their  great- 
est development  and  more  than  eleven  hundred 
species  have  been  described  from  the  rocks  of 
this  period.    It  is   interesting  to  note  that  in 
many  species  the  segments  of  the  thorax  have 
now  come  to  be  so  shaped  that  they  move  freely 
on  one  another.    Unlike  their  Cambrian  ances- 
tors, many  of   the   Ordovician   trilobites   could 
FIG  279    Crinc       t  ro^  themselves  into  balls  at  the  approach  of 
Jurassic  Species      danger.     It  is  in    this    attitude,   taken    at   the 


THE  ORDOVICIAN  AND  SILURIAN 


333 


approach  of  death,  that  trilobites  are  often  found  in  the  Ordo- 
vician  and  later  rocks.  The  gigantic  crustaceans  called  the 
eurypterids  were  also  present  in  this 
period  (Fig.  282). 


FIG.  280.  An  Ordo- 
vician  Starfish 


FIG.  281.  An  Ordovi- 
cian  Sea  Urchin 


FIG.  282.   Eurypterus 


The  arthropods  had  now  seized  upon  the  land.  Centipedes 
and  insects  of  a  low  type,  the  earliest  known  land  animals,  have 
been  discovered  in  strata  of  this  system. 

Bryozoans.  No  fossils  are  more  common 
in  the  limestones  of  the  time  than  the  small 
branching  stems  and  lacelike  mats  of  the 
bryozoans,  —  the  skeletons  of  colonies  of  a 
minute  animal  allied  in  structure  to  the 
brachiopod. 

Brachiopods.    These   multiplied    greatly, 
and  in  places  their  shells  formed  thick  beds  of  coquina.    They 
still  greatly  surpassed  the  mollusks  in  numbers. 


FIG.  283.   A  Bryozoan 


FIG.  284.    Ordovician  Brachiopods 

Cephalopods.    Among  the  mollusks  we  must  note  the  evolu- 
tion of  the  cephalopods.    The  primitive  straight  Orthoceras  has 


334 


THE   ELEMENTS  OF   GEOLOGY 


ABC 
FIG.  285.  A,  Cyrtoceras;  B,  Trochoceras; 
(7,  Lituites 


now  become  abundant.    But  in  addition  to  this  ancestral  type 
there  appears  a  succession  of  forms  more  and  more  curved  and 

closely  coiled,  as 
illustrated  in  Figure 
285.  The  nautilus, 
which  began  its 
course  in  this 
period,  crawls  on 
the  bottom  of  our 
present  seas. 

Vertebrates.  The 
most  important 
record  of  the  Ordo- 
vician  is  that  of  the 
appearance  of  a  new  and  higher  type,  with  possibilities  of 
development  lying  hidden  in  its  structure  that  the  mollusk 
and  the  insect  could  never  hope  to  reach.  Scales  and  plates 
of  minute  fishes  found  in  the  Ordovician  rocks  near  Canon 
City,  Colorado,  show  that  the  hum- 
blest of  the  vertebrates  had  already 
made  its  appearance.  But  it  is  prob- 
able that  vertebrates  had  been  on 
the  earth  for  ages  before  this  in 
lowly  types,  which,  being  destitute 
of  hard  parts,  would  leave  no  record. 

THE  SILURIAN 

The  narrowing  of  the  seas  and  the 
emergence  of  the  lands  which  char- 
acterized the  closing  epoch  of  the 

FIG.  286.   Nautilus 
Ordovician  in  eastern  North  America 

continue  into  the  succeeding  period  of  the  Silurian.    New  spe- 
cies appear  and  many  old  species  now  become  extinct. 


THE   ORDOVICIAN  AND  SILURIAN  335 

The  Appalachian  region.  Where  the  Silurian  system  is  most 
fully  developed,  from  New  York  southward  along  the  Appala- 
chian Mountains,  it  comprises  four  series : 

4  Salina  .  . N  .  shales,  impure  limestones,  gypsum,  salt 

3  Niagara  .  .  .  chiefly  limestones 

2  Clinton  .  .  .  sandstones,  shales,  with  some  limestones 

1  Medina  .  .  .  conglomerates,  sandstones 

The  rocks  of  these  series  are  shallow-water  deposits  and 
reach  the  total  thickness  of  some  five  thousand  feet.  Evidently 
they  were  laid  over  an  area  which  was  on  the  whole  gradually 
subsiding,  although  with  various  gentle  oscillations  which  are 
recorded  in  the  different  formations.  The  coarse  sands  of  the 
heavy  Medina  formations  record  a  period  of  uplift  of  the  old- 
land  of  Appalachia,  when  erosion  went  on  rapidly  and  coarse 
waste  in  abundance  was  brought  down  from  the  hills  by  swift 
streams  and  spread  by  the  waves  in  wide,  sandy  flats.  As  the 
lands  were  worn  lower  the  waste  became  finer,  and  during 
an  epoch  of  transition  —  the  Clinton  —  there  were  deposited 
various  formations  of  sandstones,  shales,  and  limestones.  The 
Niagara  limestones  testify  to  a  long  epoch  of  repose,  when  low- 
lying  lands  sent  little  waste  down  to  the  sea. 

The  gypsum  and  salt  deposits  of  the  Salina  show  that  toward 
the  close  of  the  Silurian  period  a  slight  oscillation  brought  the 
sea  floor  nearer  to  the  surface,  and  at  the  north  cut  off  exten- 
sive tracts  from  the  interior  sea.  In  these  wide  lagoons,  which 
now  and  then  regained  access  to  the  open  sea  and  obtained  new 
supplies  of  salt  water,  beds  of  salt  and  gypsum  were  deposited 
as  the  briny  waters  became  concentrated  by  evaporation  under 
a  desert  climate.  Along  with  these  beds  there  were  also  laid 
shales  and  impure  limestones. 

In  New  York  the  "  salt  pans  "  of  the  Salina  extended  over  an  area 
one  hundred  and  fifty  miles  long  from  east  to  west  and  sixty  miles  wide, 
and  similar  salt  marshes  occurred  as  far  west  as  Cleveland,  Ohio,  and 


336         THE  ELEMENTS  OF  GEOLOGY 

Goderich  on  Lake  Huron.  At  Ithaca,  New  York,  the  series  is  fifteen  hun- 
dred feet  thick,  and  is  buried  beneath  an  equal  thickness  of  later  strata. 
It  includes  two  hundred  and  fifty  feet  of  solid  salt,  in  several  distinct 
beds,  each  sealed  within  the  shales  of  the  series. 

Would  you  expect  to  find  ancient  beds  of  rock  salt  inclosed  in  beds 
of  pervious  sandstone  ? 

The  salt  beds  of  the  Salina  are  of  great  value.  They  are  reached  by 
well  borings,  and  their  brines  are  evaporated  by  solar  heat  and  by  boil- 
ing. The  rock  salt  is  also  mined  from  deep  shafts. 

Similar  deposits  of  salt,  formed  under  like  conditions,  occur  in  the 
rocks  of  later  systems  down  to  the  present.  The  salt  beds  of  Texas  are 
Permian,  those  of  Kansas  are  Permian,  and  those  of  Louisiana  are 
Tertiary. 

The  Mississippi  valley.  The  heavy  near-shore  formations  of 
the  Silurian  in  the  Appalachian  region  thin  out  toward  the  west. 
The  Medina  and  the  Clinton  sandstones  are  not  found  west  of 
Ohio,  where  the  first  passes  into  a  shale  and  the  second  into  a 
limestone.  The  Niagara  limestone,  however,  spreads  from  the 
Hudson  Eiver  to  beyond  the  Mississippi,  a  distance  of  more  than 
a  thousand  miles.  During  the  Silurian  period  the  Mississippi 
valley  region  was  covered  with  a  quiet,  shallow,  limestone- 
making  sea,  which  received  little  waste  from  the  low  lands 
which  bordered  it. 

The  probable  distribution  of  land  and  sea  in  eastern  North 
America  and  western  Europe  is  shown  in  Figure  287.  The 
fauna  of  the  interior  region  and  of  eastern  Canada  are  closely 
allied  with  that  of  western  Europe,  and  several  species  are 
identical.  We  can  hardly  account  for  this  except  by  a  shallow- 
water  connection  between  the  two  ancient  epicontinental  seas. 
It  was  perhaps  along  the  coastal  shelves  of  a  northern  land  con- 
necting America  and  Europe  by  way  of  Greenland  and  Iceland 
that  the  migration  took  place,  so  that  the  same  species  came  to 
live  in  Iowa  and  in  Sweden. 

The  western  United  States.  So  little  is  found  of  the  rocks  of 
the  system  west  of  the  Missouri  Eiver  that  it  is  quite  probable 


THE  ORDOVICIAN  AND  SILURIAN 


337 


that  the  western  part  of  the  United  States  had  for  the  most 
part  emerged  from  the  sea  at  the  close  of  the  Ordovician  and 


FIG.  287.   Hypothetical  Map  of  Parts  of  North  America  and 
Europe  in  Silurian  Time 

Shaded  areas,  probable  seas ;  broken  lines,  approximate 
shore  lines 

remained   land    during  the   Silurian.    At  the   same  time  the 
western  land  was  perhaps  connected  with  the  eastern  land  of 


FIG.  288.   A  Compound  Cup  Coral 


FIG.  289.  A  Simple 
Cup  Coral 


Appalachia  across  Arkansas  and  Mississippi;   for  toward  the 
south  the  Silurian  sediments  indicate  an  approach  to  shore. 


338 


THE   ELEMENTS  OF  GEOLOGY 

Life  of  the  Silurian 


In  this  brief  sketch  it  is  quite  impossible  to  relate  the  many 
changes  of  species  and  genera  during  the  Silurian. 

Corals.  Some  of  the  more  common  types  are  familiarly 
known  as  cup  corals,  honeycomb  corals,  and  chain  corals.  In 


FIG.  290.   Honeycomb  Corals 

the  cup  corals  the  most  important  feature  is  the  development 
of  radiating  vertical  partitions,  or  septa,  in  the  cell  of  the  polyp. 
Some  of  the  cup  corals  grew  in  hemispherical  colonies  (Fig.  288), 
while  many  were  separate  individuals  (Fig.  289),  building  a 


FIG.  291.   A  Chain  Coral  FIG.  292.   A  Syringopora  Coral 

single  conical,  or  horn-shaped  cell,  which  sometimes  reached 
the  extreme  size  of  a  foot  in  length  and  two  or  three  inches  in 
diameter. 


THE  ORDOVICIAN  AND  SILURIAN 


339 


Honeycomb  corals  consist  of  masses  of  small,  close-set  pris- 
matic cells,  each  crossed  by  horizontal  partitions,  or  tabulae, 
while  the  septa  are  rudimentary,  being  represented  by  faintly 
projecting  ridges  or  rows  of  spines. 

Chain  corals  are  also  marked  by  tabulae.    Their  cells  form 
elliptical  tubes,  touching  each  other  at  the  edges,  and  appearing 
in  cross  section  like  the  links 
of  a  chain.    They  became  ex- 
tinct at  the  end  of  the  Silurian. 

The  corals  of  the  Syrinyo- 
pora    family    are    similar   in 


B 


FIG.  293.  A  Blastoid  :  A,  side  view, 
showing  portion  of  the  stem ; 
B,  summit  of  calyx  (species  Car- 
boniferous) 


FIG.  294.   A  Silurian  Scorpion 


structure  to  chain  corals,  but  the  tubular  columns  are  con- 
nected only  in  places. 

To  the  echinoderms  there  is  now  added  the  Uastoid  (bud- 
shaped).  The  blastoid  is  stemmed  and  armless,  and  its  globular 
"  head  "  or  "  calyx,"  with  its  five  petal-like  divisions,  resembles 
a  flower  bud.  The  blastoids  became  more  abundant  in  the 
Devonian,  culminated  in  the  Carboniferous,  and  disappeared  at 
the  end  of  the  Paleozoic. 

The  great  eurypterids  —  some  of  which  were  five  or  six  feet 
in  length  —  and  the  cephalopods  were  still  masters  of  the  seas. 


340          THE  ELEMENTS  OF  GEOLOGY 

Fishes  were  as  yet  few  and  small ;  trilobites  and  graptolites 
had  now  passed  their  prime  and  had  diminished  greatly  in  num- 
bers. Scorpions  are  found  in  this  period  both  in  Europe  and  in 
America.  The  limestone-making  seas  of  the  Silurian  swarmed 
with  corals,  crinoids,  and  brachiopods. 


FIG.  295.   Block  of  Limestone  showing  Interior  Casts  of  Penlamerus 
oblongus,  a  Common  Silurian  Brachiopod 

With  the  end  of  the  Silurian  period  the  Age  of  Invertebrates 
comes  to  a  close,  giving  place  to  the  Devonian,  the  Age  of 
Fishes. 


CHAPTEE  XVIII 
THE  DEVONIAN 

In  America  the  Silurian  is  not  separated  from  the  Devonian 
by  any  mountain-making  deformation  or  continental  uplift.  The 
one  period  passed  quietly  into  the  other.  Their  conformable 
systems  are  so  closely  related,  and  the  change  in  their  faunas 
is  so  gradual,  that  geologists  are  not  agreed  as  to  the  precise 
horizon  which  divides  them. 

Subdivisions  and  physical  geography.  The  Devonian  is  rep- 
resented in  New  York  and  southward  by  the  following  five 
series.  We  add  the  rocks  of  which  they  are  chiefly  composed. 

5  Chemung  sandstones  and  sandy  shales 

4  Hamilton  shales  and  sandstones 

3  Corniferous limestones 

2  Oriskany  sandstones 

1  Helderberg limestones 

The  Helderberg  is  a  transition  epoch  referred  by  some  geol- 
ogists to  the  Silurian.  The  thin  sandstones  of  the  Oriskany 
mark  an  epoch  when  waves  worked  over  the  deposits  of  for- 
mer coastal  plains.  The  limestones  of  the  Corniferous  testify 
to  a  warm  and  clear  wide  sea  which  extended  from  the  Hud- 
son to  beyond  the  Mississippi.  Corals  throve  luxuriantly,  and 
their  remains,  with  those  of  mollusks  and  other  lime-secret- 
ing animals,  built  up  great  beds  of  limestone.  The  bordering 
continents,  as  during  the  later  Silurian,  must  now  have  been 
monotonous  lowlands  which  sent  down  little  of  even  the  finest 
waste  to  the  sea. 

In  the  Hamilton  the  clear  seas  of  the  previous  epoch  became 
clouded  with  mud.  The  immense  deposits  of  coarse  sandstones 

341 


342         THE  ELEMENTS  OF  GEOLOGY 

and  sandy  shales  of  the  Chemung,  which  are  found  off  what  was 
at  the  time  the  west  coast  of  Appalachia,  prove  an  uplift  of 
that  ancient  continent. 

The  Chemung  series  extends  from  the  Catskill  Mountains  to  north- 
eastern Ohio  and  south  to  northeastern  Tennessee,  covering  an  area  of 
not  less  than  a  hundred  thousand  square  miles.  In  eastern  New  York  it 
attains  three  thousand  feet  in  thickness ;  in  Pennsylvania  it  reaches 
the  enormous  thickness  of  two  miles ;  but  it  rapidly  thins  to  the  west. 
Everywhere  the  Chemung  is  made  of  thin  beds  of  rapidly  alternating 
coarse  and  fine  sands  and  clays,  with  an  occasional  pebble  layer,  and 
hence  is  a  shallow-water  deposit.  The  fine  material  has  not  been  thor- 
oughly winnowed  from  the  coarse  by  the  long  action  of  strong  waves 
and  tides.  The  sands  and  clays  have  undergone  little  more  sorting  than 
is  done  by  rivers.  We  must  regard  the  Chemung  sandstones  as  deposits 
made  at  the  mouths  of  swift,  turbid  rivers  in  such  great  amount  that 
they  could  be  little  sorted  and  distributed  by  waves. 

Over  considerable  areas  the  Chemung  sandstones  bear  little  or  no 
trace  of  the  action  of  the  sea.  The  Catskill  Mountains,  for  example, 
have  as  their  summit  layers  some  three  thousand  feet  of  coarse  red 
sandstones  of  this  series,  whose  structure  is  that  of  river  deposits,  and 
whose  few  fossils  are  chiefly  of  fresh-water  types.  The  Chemung  is 
therefore  composed  of  delta  deposits,  more  or  less  worked  over  by  the 
sea.  The  bulk  of  the  Chemung  equals  that  of  the  Sierra  Nevada  Moun- 
tains. To  furnish  this  immense  volume  of  sediment  a  great  mountain 
range,  or  highland,  must  have  been  upheaved  where  the  Appalachian 
lowland  long  had  been.  To  what  height  the  Devonian  mountains  of 
Appalachia  attained  cannot  be  told  from  the  volume  of  the  sediments 
wasted  from  them,  for  they  may  have  risen  but  little  faster  than  they 
were  worn  down  by  denudation.  We  may  infer  from  the  character  of 
the  waste  which  they  furnished  to  the  Chemung  shores  that  they  did 
not  reach  an  Alpine  height.  The  grains  of  the  Chemung  sandstones 
are  not  those  which  would  result  from  mechanical  disintegration,  as  by 
frost  on  high  mountain  peaks,  but  are  rather  those  which  would  be  left 
from  the  long  chemical  decay  of  siliceous  crystalline  rocks  ;  for  the  more 
soluble  minerals  are  largely  wanting.  The  red  color  of  much  of  the 
deposits  points  to  the  same  conclusion.  Red  residual  clays  accumulated 
on  the  mountain  sides  and  upland  summits,  and  were  washed  as  ocherous 
silt  to  mingle  with  the  delta  sands.  The  iron-bearing  igneous  rocks 


THE  DEVONIAN  343 

of  the  oldland  also  contributed  by  their  decay  iron  in  solution  to  the 
rivers,  to  be  deposited  in  films  of  iron  oxide  about  the  quartz  grains  of 
the  Chemung  sandstones,  giving  them  their  reddish  tints. 

LIFE  OF  THE  DEVONIAN 

Plants.  The  lands  were  probably  clad  with  verdure  during 
Silurian  times,  if  not  still  earlier ;  for  some  rare  remains  of  ferns 
and  other  lowly  types  of  vegetation  have  been  found  in  the 
strata  of  that  system.  But  it  is  in  the  Devonian  that  we  dis- 
cover for  the  first  time  the  remains  of  extensive  and  luxuriant 
forests.  This  rich  flora  reached  its  climax  in  the  Carboniferous, 
and  it  will  be  more  convenient  to  describe  its  varied  types  in 
the  next  chapter. 

Rhizocarps.  In  the  shales  of  the  Devonian  are  found  micro- 
scopic spores  of  rhizocarps  in  such  countless  numbers  that  their 
weight  must  be  reckoned  in  hundreds  of  millions  of  tons.  It 
would  seem  that  these  aquatic  plants  culminated  in  this  period, 
and  in  widely  distant  portions  of  the  earth  swampy  flats  and 
shallow  lagoons  were  filled  with  vegetation  of  this  humble  type, 
either  growing  from  the  bottom  or  floating  free  upon  the  sur- 
face. It  is  to  the  resinous  spores  of  the  rhizocarps  that  the 
petroleum  and  natural  gas  from  Devonian  rocks  are  largely 
due.  The  decomposition  of  the  spores  has  made  the  shales 
highly  bituminous,  and  the  oil  and  gas  have  accumulated  in 
the  reservoirs  of  overlying  porous  sandstones. 

Invertebrates.  We  must  pass  over  the  ever-changing  groups 
of  the  invertebrates  with  the  briefest  notice.  Chain  corals 
became  extinct  at  the  close  of  the  Silurian,  but  other  corals 
were  extremely  common  in  the  Devonian  seas.  At  many 
places  corals  formed  thin  reefs,  as  at  Louisville,  Kentucky, 
where  the  hardness  of  the  reef  rock  is  one  of  the  causes  of  the 
Falls  of  the  Ohio. 

Sponges,  echinoderms,  brachiopods,  and  mollusks  were  abun- 
dant. The  cephalopods  take  a  new  departure.  So  far  in  all 


344  THE  ELEMENTS  OF  GEOLOGY 

their  various  forms,  whether  straight,  as  the  Orthoceras,  or 
curved,  or  close-coiled  as  in  the  nautilus,  the  septum,  or  parti- 
tion dividing  the  chambers,  met  the  inner  shell  along  a  simple 
line,  like  that  of  the  rim  of  a  saucer.  There  now  begins  a 

growth  of  the  septum  by  which  its 
edges  become  sharply  corrugated,  and 
the  suture,  or  line  of  juncture  of  the 
septum  and  the  shell,  is  thus  angled. 
The  group  in  which  this  growth  of 
the  septum  takes  place  is  called  the 
Goniatite  (Greek  gonia,  angle). 

Vertebrates.  It  is  with  the  great- 
est interest  that  we  turn  now  to 
study  the  backboned  animals  of  the 
Devonian;  for  they  are  believed  to 

be  the  ancestors  of  the  hosts  of  verte- 
FIG.  296.   A  Goniatite 

brates  which  have  since  dominated 

the  earth.  Their  rudimentary  structures  foreshadowed  what 
their  descendants  were  to  be,  and  give  some  clue  to  the  earliest 
vertebrates  from  which  they  sprang.  Like  those  whose  remains 
are  found  in  the  lower  Paleozoic  systems,  all  of 
these  Devonian  vertebrates  were  aquatic  and  go 
under  the  general  designation  of  fishes. 

The  lowest  in  grade  and  nearest,  perhaps,  to  the 
ancestral  type  of  vertebrates,  was  the  problematic 
creature,  an  inch  or  so  long,  of  Figure  297.  Note 
the  circular  mouth  not  supplied  with  jaws,  the  lack 
of  paired  fins,  and  the  symmetric  tail  fin,  with 
the  column  of  cartilaginous,  ringlike  vertebrse  run-  FIG.  297. 
ning  through  it  to  the  end.  The  animal  is  prob- 
ably to  be  placed  with  the  jawless  lampreys  and 
hags,  —  a  group  too  low  to  be  included  among  true  fishes. 

Ostracoderms.    This    archaic   group,   long  since  extinct,  is 
also  too  lowly  to  rank  among  the  true  fishes,  for  its  members 


THE  DEVONIAN  345 

have  neither  jaws  nor  paired  fins.  These  small,  fishlike  forms 
were  eased  in  front  with  bony  plates  developed  in  the  skin  and 
covered  in  the  rear  with  scales.  The  vertebrae  were  not  ossified, 
for  no  trace  of  them  has  been  found. 


FIG.  298.  An  Ostracoderm 

Devonian  fishes.  The  true  fishes  of  the  Devonian  can  best 
be  understood  by  reference  to  their  descendants  now  living. 
Modern  fishes  are  divided  into  several  groups:  sharks  and 
their  allies ;  dipnoans;  ganoids,  such  as  the  sturgeon  and  gar ; 
and  teleosts,  —  most  common  fishes,  such  as  the  perch  and  cod. 

Sharks.  Of  all  groups  of  living  fishes  the  sharks  are  the 
oldest  and  still  retain  most  fully  the  embryonic  characters  of 
their  Paleozoic  ancestors.  Such  characters  are  the  cartilaginous 


FIG.  299.   A  Paleozoic  Shark 

skeleton,  and  the  separate  gill  slits  with  which  the  throat  wall 
is  pierced  and  which  are  arranged  in  line  like  the  gill  openings 
of  the  lamprey.  The  sharks  of  the  Silurian  and  Devonian  are 
known  to  us  chiefly  by  their  teeth  and  fin  spines,  for  they  were 
unprotected  by  scales  or  plates,  and  were  devoid  of  a  bony 


346  THE  ELEMENTS  OF  GEOLOGY 

skeleton.  Figure  299  is  a  restoration  of  an  archaic  shark  from 
a  somewhat  higher  horizon.  Note  the  seven  gill  slits  and  the 
lappetlike  paired  fins.  These  fins  seem  to  be  remnants  of  the 
continuous  fold  of  skin  which,  as  embryology  teaches,  passed 
from  fore  to  aft  down  each  side  of  the  primitive  vertebrate. 

Devonian  sharks  were  comparatively  small.  They  had  not 
evolved  into  the  ferocious  monsters  which  were  later  to  be 
masters  of  the  seas. 

Dipnoans,  or  lung  fishes.  These  are  represented  to-day  by  a 
few  peculiar  fishes  and  are  distinguished  by  some  high  structures 
which  ally  them  with  amphibians.  An  air  sac  with  cellular 
spaces  is  connected  with  the  gullet  and  serves  as  a  rudimen- 
tary lung.  It  corresponds  with  the  swim  bladder  of  most  mod- 


FIG.  300.   A  Devonian  Dipnoan 

ern  fishes,  and  appears  to  have  had  a  common  origin  with  it. 
We  may  conceive  that  the  primordial  fishes  not  only  had  gills 
used  in  breathing  air  dissolved  in  water,  but  also  developed  a 
saclike  pouch  off  the  gullet.  This  sac  evolved  along  two  dis- 
tinct lines.  On  the  line  of  the  ancestry  of  most  modern  fishes 
its  duct  was  closed  and  it  became  the  swim  bladder  used  in 
flotation  and  balancing.  On  another  line  of  descent  it  was  left 
open,  air  was  swallowed  into  it,  and  it  developed  into  the  rudi- 
mentary lung  of  the  dipnoans  and  into  the  more  perfect  lungs 
of  the  amphibians  and  other  air-breathing  vertebrates. 

One  of  the  ancient  dipnoans  is  illustrated  in  Figure  300. 
Some  of  the  members  of  this  order  were,  like  the  ostraco- 
derms,  cased  in  armor,  but  their  higher  rank  is  shown  by  their 
powerful  jaws  and  by  other  structures.  Some  of  these  armored 


THE  DEVONIAN  347 

fishes  reached  twenty-five  feet  in  length  and  six  feet  across  the 
head.    They  were  the  tyrants  of  the  Devonian  seas. 

Ganoids.  These  take  their  name  from  their  enameled  plates 
or  scales  of  bone.  The  few  genera  now  surviving  are  the 
descendants  of  the  tribes  which  swarmed  in  the  Devonian  seas. 
A  restoration  of  one  of  a  leading  order,  the  fringe-finned 
ganoids,  is  given  in  Figure  301.  The  side  fins,  which  corre- 
spond to  the  limbs  of  the  higher  vertebrates,  are  quite  unlike 
those  of  most  modern  fishes.  Their  rays,  instead  of  radiating 
from  a  common  base,  fringe  a  central  lobe  which  contains  a 


FIG.  301.   A  Devonian  Fringe-Finned  Ganoid 

cartilaginous  axis.  The  teeth  of  the  Devonian  ganoids  show 
a  complicated  folded  structure. 

General  characteristics  of  Devonian  fishes.  The  notochord 
is  persistent.  The  notochord  is  a  continuous  rod  of  cartilage,  or 
gristle,  which  in  the  embryological  growth  of  vertebrate  ani- 
mals supports  the  spinal  nerve  cord  before  the  formation  of 
the  vertebrae.  In  most  modern  fishes  and  in  all  higher  verte- 
brates the  notochord  is  gradually  removed  as  the  bodies  of  the 
vertebrae  are  formed  about  it ;  but  in  the  Devonian  fishes  it 
persists  through  maturity  and  the  vertebrae  remain  incomplete. 

The  skeleton  is  cartilaginous.  This  also  is  an  embryological 
characteristic.  In  the  Devonian  fishes  the  vertebrae,  as  well  as 
the  other  parts  of  the  skeleton,  have  not  ossified,  or  changed  to 
bone,  but  remain  in  their  primitive  cartilaginous  condition. 


348 


THE  ELEMENTS  OF  GEOLOGY 


The  tail  fin  is  vertebrated.  The  backbone  runs  through  the 
fin  and  is  fringed  above  and  below  with  its  vertical  rays.  In 
some  fishes  with  vertebrated  tail  fins  the  fin  is  symmetric 
(Fig.  300),  and  this  seems  to  be  the  primitive  type.  In  others 
the  tail  fin  is  unsymmetric :  the  backbone  runs  into  the  upper 
lobe,  leaving  the  two  lobes  of  unequal  size.  In  most  modern 

fishes  (the  teleosts) 
the  tail  fin  is  not 
vertebrated:  the 
spinal  column  ends 
in  a  broad  plate,  to 
which  the  diver- 
ging fin  rays  are 
attached. 

But  along  with 
these  embryonic 
characters,  which 
were  common  to  all 
Devonian  fishes, 
there  were  other 
structures  in  certain  groups  which  foreshadowed  the  higher 
structures  of  the  land  vertebrates  which  were  yet  to  come :  air 
sacs  which  were  to  develop  into  lungs,  and  cartilaginous  axes  in 
the  side  fins  which  were  a  prophecy  of  limbs.  The  vertebrates 
had  already  advanced  far  enough  to  prove  the  superiority  of 
their  type  of  structure  to  all  others.  Their  internal  skeleton 
afforded  the  best  attachment  for  muscles  and  enabled  them  to 
become  the  largest  and  most  powerful  creatures  of  the  time. 
The  central  nervous  system,  with  the  predominance  given  to  the 
ganglia  at  the  fore  end  of  the  nerve  cord,  —  the  brain,  —  already 
endowed  them  with  greater  energy  than  the  invertebrates ;  and, 
still  more  important,  these  structures  contained  the  possibility 
of  development  into  the  more  highly  organized  land  vertebrates 
which  were  to  rule  the  earth. 


FIG.  302.  Vertebrae  of  Sturgeon  in  side  view  A  ; 
and  vertical  transverse  section  B,  showing 
Notochord  ch,  and  Neural  Canal  m 


THE  DEVONIAN  349 

Teleosts.  The  great  group  of  fishes  called  the  teleosts,  or  those  with 
complete  bony  skeletons,  to  which  most  modern  fishes  belong,  may  be 
mentioned  here,  although  in  the  Devonian  they  had  not  yet  appeared. 
The  teleosts  are  a  highly  specialized  type,  adapted  most  perfectly  to 
their  aquatic  environment.  Heavy  armor  has  been  discarded,  and 
reliance  is  placed  on  swiftness  instead.  The  skeleton  is  completely 
ossified  and  the  notochord  removed.  The  vertebrae  have  been  eco- 
nomically withdrawn  from  the  tail,  and  the  cartilaginous  axis  of  the 
side  fins  has  been  found  unnecessary.  The  air  sac  has  become  a  swim 
bladder.  In  this  complete  specialization  they  have  long  since  lost  the 
possibility  of  evolving  into  higher  types. 

It  is  interesting  to  note  that  the  modern  teleosts  in  their  embryo- 
logical  growth  pass  through  the  stages  which  characterized  the  maturity 
of  their  Devonian  ancestors ;  their  skeleton  is  cartilaginous  and  their 
tail  fin  vertebrated. 

Vertebrates  come  to  land.  A  footprint,  the  oldest  known, 
found  in  Upper  Devonian  rocks  of  Pennsylvania,  shows  that 
vertebrates  then  trod  the  land,  and  fishes  had  developed  into 
amphibians.  The  goal  of  the  land  was  won,  it  seems,  by  the 
fringe-filmed  ganoids,  who  lived  not  in  the  sea  but  in  the 
streams  and  lakes  of  semi-arid  regions.  In  dry  seasons  these 
streams  dwindled  to  stagnant  pools  or  withered  quite  away. 
Hence  these  fishes  were  compelled  to  use  their  air  sacs  more 
and  more  as  lungs,  gulping  down  air  to  supplement  that  taken 
by  their  gills  from  the  stagnant,  overcrowded  water.  Mean- 
while they  were  able  to  crawl  about,  when  streams  went  dry, 
by  means  of  their  side  fins,  whose  cartilaginous  supports  were 
developing  into  strong  limb  bones.  The  Devonian  fringe-finned 
ganoids  are  supposed  to  have  been  the  ancestors  of  the  first 
amphibia,  not  only  because  their  fins  and  air  sacs  could  be 
adapted  to  life  on  land,  but  also  because  they  wrere  very  like 
early  amphibia  in  the  bones  of  the  skull  and  the  infolded 
structure  of  the  teeth. 


CHAPTEE  XIX 
THE  CARBONIFEROUS 

The  Carboniferous  system  is  so  named  from  the  large  amount 
of  coal  which  it  contains.  Other  systems,  from  the  Devonian 
on,  are  coal  bearing  also,  but  none  so  richly  and  to  so  wide  an 
extent.  Never  before  or  since  have  the  peculiar  conditions  been 
so  favorable  for  the  formation  of  extensive  coal  deposits. 

With  few  exceptions  the  Carboniferous  strata  rest  on  those 
of  the  Devonian  without  any  marked  unconformity ;  the  one 
period  passed  quietly  into  the  other,  with  no  great  physical 
disturbances. 

The  Carboniferous  includes  three  distinct  series.  The  lower  is 
called  the  Mississippian,  from  the  outcrop  of  its  formations  along 
the  Mississippi  River  in  central  and  southern  Illinois  and  the 
adjacent  portions  of  Iowa  and  Missouri.  The  middle  series  is 
called  the  Pennsylvanian  (or  Coal  Measures),  from  its  wide 
occurrence  over  Pennsylvania.  The  upper  series  is  named  the 
Permian,  from  the  province  of  Perm  in  Russia. 

The  Mississippian  series.  In  the  interior  the  Mississippian 
is  composed  chiefly  of  limestones,  with  some  shales,  which  tell 
of  a  clear,  warm,  epicontinental  sea  swarming  with  crinoids, 
corals,  and  shells,  and  occasionally  clouded  with  silt  from  the 
land. 

In  the  eastern  region,  New  York  had  been  added  by  uplift 
to  the  Appalachian  land  which  now  was  united  to  the  northern 
area.  From  eastern  Pennsylvania  southward  there  were  laid  in  a 
subsiding  trough,  first,  thick  sandstones  (the  Pocono  sandstone), 
and  later  still  heavier  shales,  —  the  two  together  reaching  the 
thickness  of  four  thousand  feet  and  more.  We  infer  a  renewed 

350 


rTHE  CARBONIFEROUS  351 

uplift  of  Appalachia  similar  to  that  of  the  later  epochs  of  the 
Devonian,  but  as  much  less  in  amount  as  the  volume  of  sedi- 
ments is  smaller. 

THE  PENNSYLVANIA^  SEEIES 

The  Mississippian  was  brought  to  an  end  by  a  quiet  oscilla- 
tion which  lifted  large  areas  slightly  above  the  sea,  and  the 
Pennsylvanian  began  with  a  movement  in  the  opposite  direction. 
The  sea  encroached  on  the  new  land,  and  spread  far  and  wide 
a  great  basal  conglomerate  and  coarse  sandstones.  On  this 
ancient  beach  deposit  a  group  of  strata  rests  which  we  must 
now  interpret.  They  consist  of  alternating  shales  and  sand- 
stones, with  here  and  there  a  bed  of  limestone  and  an  occa- 
sional seam  of  coal.  A  stratum  of  fire  clay  commonly  underlies 
a  coal  seam,  and  there  occur  also  beds  of  iron  ore.  We  give 
a  typical  section  of  a  very  small  portion  of  the  series  at  a  local- 
ity in  Pennyslvania.  Although  some  of  the  minor  changes  are 
omitted,  the  section  shows  the  rapid  alternation  of  the  strata : 

Feet 

9   Sandstone  and  shale 25 

8  Limestone 18 

7  Sandstone 10 

6  Coal 1-6 

5  Shale 0-2 

4  Sandstone 40 

3  Limestone 10 

2  Coal 5-12 

1  Fire  clay     ...... 3 

This  section  shows  more  coal  than  is  usual;  on  the  whole, 
coal  seams  do  not  take  up  more  than  one  foot  in  fifty  of  the 
Coal  Measures.  They  vary  also  in  thickness  more  than  is  seen 
in  the  section,  some  exceptional  seams  reaching  the  thickness 
of  fifty  feet. 


352  THE  ELEMENTS  OF  GEOLOGY 

How  coal  was  made.  1.  Coal  is  of  vegetable  origin.  Exam- 
ined under  the  microscope  even  anthracite,  or  hard  coal,  is  seen 
to  contain  carbonized  vegetal  tissues.  There  are  also  all  grada- 
tions connecting  the  hardest  anthracite  —  through  semibitumi- 
nous  coal,  bituminous  or  soft  coal,  lignite  (an  imperfect  coal 
in  which  sometimes  woody  fibers  may  be  seen  little  changed) 
—  with  peat  and  decaying  vegetable  tissues.  Coal  is  compressed 
and  mineralized  vegetal  matter.  Its  varieties  depend  on  the 
perfection  to  which  the  peculiar  change  called  bituminization 
has  been  carried,  and  also,  as  shown  in  the  table  below,  on  the 
degree  to  which  the  volatile  substances  and  water  have  escaped, 
and  on  the  per  cent  of  carbon  remaining. 

Peat  Lignite        Bituminous  Coal    Anthracite 


Dismal  Swamp        Texas 

Penn. 

Penn. 

Moisture     .     .     . 

78.89 

14.67 

1.3 

2.74 

Volatile  matter    . 

13.84 

37.32 

20.87 

4.25 

Fixed  carbon 

6.49 

41.07 

67.20 

81.51 

Ash    . 

.78 

6.69 

8.80    ' 

10.87 

2.  The  vegetable  remains  associated  with  coal  are  those  of 
land  plants. 

3.  Coal  accumulated  in  the  presence  of  water ;  for  it  is  only 
when  thus  protected  from  the  air  that  vegetal  matter  is  pre- 
served. 

4.  The  vegetation  of   coal  accumulated  for  the  most  part 
where  it  grew ;  it  was  not  generally  drifted  and  deposited  by 
waves  and  currents.    Commonly  the  fire  clay  beneath  the  seam 
is  penetrated  with  roots,  and  the  shale  above  is  packed  with 
leaves  of  ferns  and  other  plants  as  beautifully  pressed  as  in  a 
herbarium.    There  often  is  associated  with  the  seam  a  fossil 
forest,  with  the  stumps,  which  are  still  standing  where  they 
grew,  their  spreading  roots,  and  the  soil  beneath,  all  changed  to 
stone  (Fig.  303).    In  the  Nova  Scotia  field,  out  of  seventy-six 
distinct  coal  seams,  twenty  are  underlain  by  old  forest  grounds. 


THE  CARBONIFEROUS 


353 


The  presence  of  fire  clay  beneath  a  seam  points  in  the  same 
direction.  Such  underclays  withstand  intense  heat  and  are  used 
in  making  fire  brick,  because  their  alkalies  have  been  removed 
by  the  long-continued  growth  of  vegetation. 

Fuel  coal  is  also  too  pure  to  have  been  accumulated  by 
driftage.  In  that  case  we  should  expect  to  find  it  mixed  with 
mud,  while  in  fact  it  often  contains  no  more  ash  than  the 
vegetal  matter  would  furnish  from  which  it  has  been  compressed. 


FIG.  303.  Fossil  Tree  Stumps  of  a  Carboniferous  Forest,  Scotland 

These  conditions  are  fairly  met  in  the  great  swamps  of  river 
ins  and  deltas  and  of  coastal  plains,  such  as  the  great  Dismal 
Swamp,  where  thousands  of  generations  of  forests  with  their 
ergrowths  contribute  their  stems  and  leaves  to  form  thick 
beds  of  peat.  A  coal  seam  is  a  fossil  peat  bed. 

Geographical  conditions  during  the  Pennsylvanian.    The  Car- 
iferous  peat  swamps  were  of  vast  extent.    A  map  of  the 
Coal  Measures  (Fig.  260)  shows  that  the  coal  marshes  stretched, 
with  various  interruptions  of  higher  ground  and  straits  of  open 


35-4  THE  ELEMENTS  OF  GEOLOGY 

water,  from  eastern  Pennsylvania  into  Alabama,  Texas,  and 
Kansas.  Some  individual  coal  beds  may  still  be  traced  over  a 
thousand  square  miles,  despite  the  erosion  which  they  have 
suffered.  It  taxes  the  imagination  to  conceive  that  the  varied 
region  included  within  these  limits  was  for  hundreds  of  thou- 
sands of  years  a  marshy  plain  covered  with  tropical  jungles 
such  as  that  pictured  in  Figure  304. 

On  the  basis  that  peat  loses  four  fifths  of  its  bulk  in  chan- 
ging to  coal,  we  may  reckon  the  thickness  of  these  ancient  peat 
beds.  Coal  seams  six  and  ten  feet  thick,  which  are  not  uncom- 
mon, represent  peat  beds  thirty  and  fifty  feet  in  thickness, 
while  mammoth  coal  seams  fifty  feet  thick  have  been  com- 
pressed from  peat  beds  two  hundred  and  fifty  feet  deep. 

At  the  same  time,  the  thousands  of  feet  of  marine  and  fresh- 
water sediments,  with  their  repeated  alternations  of  limestones, 
sandstones,  and  shales,  in  which  the  seams  of  coal  occur,  prove 
a  slow  subsidence,  with  many  changes  in  its  rate,  with  halts 
when  the  land  was  at  a  stillstand,  and  with  occasional  move- 
ments upward. 

When  subsidence  was  most  rapid  and  long  continued  the 
sea  encroached  far  and  wide  upon  the  lowlands  and  covered  the 
coal  swamps  with  sands  and  muds  and  limy  oozes.  When  sub- 
sidence slackened  or  ceased  the  land  gained  on  the  sea.  Bays 
were  barred,  and  lagoons  as  they  gradually  filled  with  mud 
became  marshes.  Eiver  deltas  pushed  forward,  burying  with 
their  silts  the  sunken  peat  beds  of  earlier  centuries,  and  at  the 
surface  emerged  in  broad,  swampy  flats,  —  like  those  of  the 
deltas  of  the  Mississippi  and  the  Ganges,  —  which  soon  were 
covered  with  luxuriant  forests.  At  times  a  gentle  uplift  brought 
to  sea  level  great  coastal  plains,  which  for  ages  remained  mantled 
with  the  jungle,  their  undeveloped  drainage  clogged  with  its 
debris,  and  were  then  again  submerged. 

Physical  geography  of  the  several  regions.  The  Acadian 
region  lay  on  the  eastern  side  of  the  northern  land,  where  now 


365 


356          THE  ELEMENTS  OF  GEOLOGY 

are  New  Brunswick  and  Nova  Scotia,  and  was  an  immense 
river  delta.  Here  river  deposits  rich  in  coal  accumulated  to  a 
depth  of  sixteen  thousand  feet.  The  area  of  this  coal  field  is 
estimated  at  about  thirty-six  thousand  square  miles. 

The  Appalachian  region  skirts  the  Appalachian  oldland  on 
the  west  from  the  southern  boundary  of  New  York  to  northern 
Alabama,  extending  west  into  eastern  Ohio.  The  Cincinnati 
anticline  was  now  a  peninsula,  and  the  broad  gulf  which  had  lain 
between  it  and  Appalachia  was  transformed  at  the  beginning  of 
the  Pennsylvanian  into  wide  marshy  plains,  now  sinking  beneath 
the  sea  and  now  emerging  from  it.  This  area  subsided  during 
the  Carboniferous  period  to  a  depth  of  nearly  ten  thousand  feet. 

The  Central  region  lay  west  of  the  peninsula  of  the  Cin- 
cinnati anticline,  and  extended  from  Indiana  west  into  eastern 
Nebraska,  and  from  central  Iowa  and  Illinois  southward  about 
the  ancient  island  in  Missouri  and  Arkansas  into  Oklahoma 
and  Texas.  On  the  north  the  subsidence  in  this  area  was  com- 
paratively slight,  for  the  Carboniferous  strata  scarcely  exceed 
two  thousand  feet  in  thickness.  But  in  Arkansas  and  Indian 
Territory  the  downward  movement  amounted  to  four  and  five 
miles,  as  is  proved  by  shoal  water  deposits  of  that  immense 
thickness. 

The  coal  fields  of  Indiana  and  Illinois  are  now  separated  by 
erosion  from  those  lying  west  of  the  Mississippi  Eiver.  At  the 
south  the  Appalachian  land  seems  still  to  have  stretched  away 
to  the  west  across  Louisiana  and  Mississippi  into  Texas,  and 
this  westward  extension  formed  the  southern  boundary  of  the 
coal  marshes  of  the  continent. 

The  three  regions  just  mentioned  include  the  chief  Carbon- 
iferous coal  fields  of  North  America.  Including  a  field  in  central 
Michigan  evidently  formed  in  an  inclosed  basin  (Fig.  260),  and 
one  in  Ehode  Island,  the  total  area  of  American  coal  fields  has 
been  reckoned  at  not  less  than  two  hundred  thousand  square 
miles.  We  can  hardly  estimate  the  value  of  these  great  stores 


THE  .CARBONIFEROUS  357 

of  fossil  fuel  to  an  industrial  civilization.  The  forests  of  the 
coal  swamps  accumulated  in  their  woody  tissues  the  energy 
which  they  received  from  the  sun  in  light  and  heat,  and  it  is 
this  solar  energy  long  stored  in  coal  seams  which  now  forms 
the  world's  chief  source  of  power  in  manufacturing. 

The  western  area.  On  the  Great  Plains  beyond  the  Missouri 
Kiver  the  Carboniferous  strata  pass  under  those  of  more  recent 
systems.  Where  they  reappear,  as  about  dissected  mountain 
axes  or  on  stripped  plateaus,  they  consist  wholly  of  marine 
deposits  and  are  devoid  of  coal.  The  rich  coal  fields  of  the  West 
are  of  later  date. 

On  the  whole  the  Carboniferous  seems  to  have  been  a  time 
of  subsidence  in  the  West.  Throughout  the  period  a  sea  covered 
the  Great  Basin  and  the  plateaus  of  the  Colorado  Eiver.  At  the 
time  of  the  greatest  depression  the  sites  of  the  central  chains  of. 
the  Rockies  were  probably  islands,  but  early  in  the  period  they 
may  have"  been  connected  with  the  broad  lands  to  the  south 
and  east.  Thousands  of  feet  of  Carboniferous  sediments  were 
deposited  where  the  Sierra  Nevada  Mountains  now  stand. 

The  Permian.  As  the  Carboniferous  period  drew  toward  its 
close  the  sea  gradually  withdrew  from  the  eastern  part  of  the 
continent.  Where  the  sea  lingered  in  the  deepest  troughs,  and 
where  inclosed  basins  were  cut  off  from  it,  the  strata  of  the  Per- 
mian were  deposited.  Such  are  found  in  New  Brunswick,  in 
Pennsylvania  and  West  Virginia,  in  Texas,  and  in  Kansas.  In 
southwestern  Kansas  extensive  Permian  beds  of  rock  salt  and 
gypsum  show  that  here  lay  great  salt  lakes  in  which  these 
minerals  were  precipitated  as  their  brines  grew  dense  and  dried 
away. 

•  In  the  southern  hemisphere  the  Permian  deposits  are  so  extraordi- 
nary that  they  deserve  a  brief  notice,  although  we  have  so  far  omitted 
mention  of  the  great  events  which  characterized  the  evolution  of  other 
continents  than  our  own.  The  Permian  fauna-flora  of  Australia,  India, 
South  Africa,  and  the  southern  part  of  South  America  are  so  similar 


358  THE  ELEMENTS  OF  GEOLOGY 

that  the  inference  is  a  reasonable  one  that  these  "widely  separated 
regions  were  then  connected  together,  probably  as  extensions  of  a  great 
antarctic  continent. 

Interbedded  with  the  Permian  strata  of  the  first  three  countries 
named  are  extensive  and  thick  deposits  of  a  peculiar  nature  which  are 
clearly  ancient  ground  moraines.  Clays  and  sand,  now  hardened  to 
firm  rock,  are  inset  with  unsorted  stones  of  all  sizes,  which  often  are 
faceted  and  scratched.  Moreover,  these  bowlder  clays  rest  on  rock 
pavements  which  are  polished  and  scored  with  glacial  markings. 
Hence  toward  the  close  of  the  Paleozoic  the  southern  lands  of  the  east- 
ern hemisphere  were  invaded  by  ice  sheets  like  that  which  now  shrouds 
Greenland. 

These  Permian  ground  moraines  are  not  the  first  traces  of  the  work 
of  glaciers  met  with  in  the  geological  record.  Similar  deposits  prove 
glaciation  in  Norway  succeeding  the  pre-Cambrian  stage  of  elevation, 
and  Cambrian  glacial  drift  has  recently  been  found  in  China  (page  314). 

The  Appalachian  deformation.  We  have  seen  that  during 
Paleozoic  times  a  long,  narrow  trough  of  the  sea  lay  off  the 
western  coast  of  the  ancient  land  of  Appalachia,  where  now  are 
the  Appalachian  Mountains.  During  the  long  ages  of  this  era 
the  trough  gradually  subsided,  although  with  many  stillstands 
and  with  occasional  slight  oscillations  upward.  Meanwhile 
the  land  lying  to  the  east  was  gradually  uplifted  at  varying 
rates  and  with  long  pauses.  The  waste  of  the  rising  land 
was  constantly  transferred  to  the  sinking  marginal  sea  bottom, 
and  on  the  whole  the  trough  was  filled  with  sediments  as 
rapidly  as  it  subsided.  The  sea  was  thus  kept  shallow,  and  at 
times,  especially  toward  the  close  of  the  era,  much  of  the  area 
was  upbuilt  or  raised  to  low,  marshy  coastal  plains.  When  the 
Carboniferous  was  ended  the  waste  which  had  been  removed 
from  the  land  and  laid  along  its  margin  in  the  successive  forma- 
tions of  the  Paleozoic  had  reached  a  thickness  of  between  thirty 
and  forty  thousand  feet. 

Both  by  sedimentation  and  by  subsidence  the  trough  had 
now  become  a  belt  of  weakness  in  the  crust  of  the  earth.  Here 


THE  CARBONIFEROUS  359 

the  crust  was  now  made  of  layers  to  the  depth  of  six  or  seven 
miles.  In  comparison  with  the  massive  crystalline  rocks  of 
Appalachia  on  the  east,  the  layered  rock  of  the  trough  was  weak 
to  resist  lateral  pressure,  as  a  ream  of  sheets  of  paper  is  weak 
when  compared  with  a  solid  board  of  the  same  thickness.  It  was 
weaker  also  than  the  region  to  the  west,  since  there  the  sedi- 
ments were  much  thinner.  Besides,  by  the  long-continued 
depression  the  strata  of  the  trough  had  been  bent  from  the 
flat-lying  attitude  in  which  they  were  laid  to  one  in  which  they 
were  less  able  to  resist  a  horizontal  thrust. 

There  now  occurred  one  of  the  critical  stages  in  the  history 
of  the  planet,  when  the  crust  crumples  under  its  own  weight 
and  shrinks  down  upon  a  nucleus  which  is  diminishing  in  vol- 
ume and  no  longer  able  to  support  it.  Under  slow  but  resist- 
less pressure  the  strata  of  the  Appalachian  trough  were  thrust 
against  the  rigid  land,  and  slowly,  steadily  bent  into  long  folds 
whose  axes  ran  northeast-southwest  parallel  to  the  ancient 
coast  line.  It  was  on  the  eastern  side  next  the  buttress  of  the 
land  that  the  deformation  was  the  greatest,  and  the  folds  most 
steep  and  close.  In  central  Pennsylvania  and  West  Virginia  the 
folds  were  for  the  most  part  open.  South  of  these  states  the  folds 
were  more  closely  appressed,  the  strata  were  much  broken,  and 
the  great  thrust  faults  were  formed  which  have  been  described 
already  (p.  218).  In  eastern  Pennsylvania  seams  of  bituminous 
coal  were  altered  to  anthracite,  while  outside  the  region  of 
strong  deformation,  as  in  western  Pennyslvania,  they  remained 
unchanged.  An  important  factor  in  the  deformation  was  the 
massive  limestones  of  the  Cambrian-Ordovician.  Because  of 
these  thick,  resistant  beds  the  rocks  were  bent  into  wide  folds 
and  sheared  in  places  with  great  thrust  faults.  Had  the  strata 
been  weak  shales,  an  equal  pressure  would  have  crushed  and 
mashed  them. 

Although  the  great  earth  folds  were  slowly  raised,  and  no 
doubt  eroded  in  their  rising,  they  formed  in  all  probability  a 


360         THE  ELEMENTS  OF  GEOLOGY 

range  of  lofty  mountains,  with  a  width  of  from  fifty  to  a  hun- 
dred and  twenty-five  miles,  which  stretched  from  New  York 
to  central  Alabama. 

From  their  bases  lowlands  extended  westward  to  beyond  the 
Missouri  Eiver.  At  the  same  time  ranges  were  upridged  out 
of  thick  Paleozoic  sediments  both  in  the  Bay  of  Fundy  region 
and  in  the  Indian  Territory.  The  eastern  portion  of  the  North 
American  continent  was  now  well-nigh  complete. 

The  date  of  the  Appalachian  deformation  is  told  in  the  usual 
way.  The  Carboniferous  strata,  nearly  two  miles  thick,  are  all 
infolded  in  the  Appalachian  ridges,  while  the  next  deposits 
found  in  this  region  —  those  of  the  later  portion  of  the  first 
period  (the  Trias)  of  the  succeeding  era  —  rest  unconf ormably  on 
the  worn  edges  of  the  Appalachian  folded  strata.  The  deforma- 
tion therefore  took  place  about  the  close  of  the  Paleozoic.  It 
seems  to  have  begun  in  the  Permian,  in  eastern  Pennsylvania, 
—  for  here  the  Permian  strata  are  wanting,  —  and  to  have  con- 
tinued into  the  Trias,  whose  earlier  formations  are  absent  over 
all  the  area. 

With  this  wide  uplift  the  subsidence  of  the  sea  floor  which 
had  so  long  been  general  in  eastern  North  America  carne  to  an 
end.  Deposition  now  gave  place  to  erosion.  The  sedimentary 
record  of  the  Paleozoic  was  closed,  and  after  an  unknown  lapse 
of  time,  here  unrecorded,  the  annals  of  the  succeeding  era  were 
written  under  changed  conditions. 

In  western  North  America  the  closing  stages  of  the  Paleozoic 
were  marked  by  important  oscillations.  The  Great  Basin,  which 
had  long  been  a  mediterranean  sea,  was  converted  into  land 
over  western  Utah  and  eastern  Nevada,  while  the  waves  of  the 
Pacific  rolled  across  California  and  western  Nevada. 

The  absence  of  tuffs  and  lavas  among  the  Carboniferous  strata 
of  North  America  shows  that  here  volcanic  action  was  singu- 
larly wanting  during  the  entire  period.  Even  the  Appalachian 
deformation  was  not  accompanied  by  any  volcanic  outbursts. 


THE  CARBONIFEROUS 


361 


LIFE  OF  THE  CARBONIFEROUS 

Plants.  The  gloomy  forests  and  dense  undergrowths  of  the 
Carboniferous  jungles  would  appear  unfamiliar  to  us  could  we 
see  them  as  they  grew,  and  even  a  botanist  would  find  many 
of  their  forms  perplexing  and  hard  to  classify.  None  of  our 
modern  trees  would  meet  the  eye.  Plants  with  conspicuous 


FIG.  305.  Carboniferous  Ferns 


FIG.  306.  Calamites 


flowers  of  fragrance  and  beauty  were  yet  to  come.  Even  mosses 
and  grasses  were  still  absent. 

Tree  ferns  lifted  their  crowns  of  feathery  fronds  high  in  air 
on  trunks  of  woody  tissue ;  and  lowly  herbaceous  ferns,  some 
belonging  to  existing  families,  carpeted  the  ground.  Many  of 
the  fernlike  forms,  however,  have  distinct  affinities  with  the 
cycads,  of  which  they  may  be  the  ancestors,  and  some  bear  seeds 
and  must  be  classed  as  gymnosperms. 

Dense  thickets,  like  cane  or  bamboo  brakes,  were  composed 
of  thick  clumps  of  Calamites,  whose  slender,  jointed  stems  shot 
up  to  a  height  of  forty  feet,  and  at  the  joints  bore  slender 


362 


THE  ELEMENTS  OF  GEOLOGY 


branches  set  with  whorls  of  leaves.  These  were  close  allies  of  the 
Equiseta  or  "  horsetails,"  of  the  present ;  but  they  bore  character- 
istics of  higher  classes  in  the  woody  structures  of  their  stems. 

There  were  also  vast  monotonous  forests,  composed  chiefly  of 
trees  belonging  to  the  lycopods,  and  whose  nearest  relatives 
to-day  are  the  little  club  mosses  of  our  eastern  woods.  Two 


FIG.  307.   Lepidodendron 


FIG.  308.   Sigillaria 


families  of  lycopods  deserve  special  mention,  —  the  Lepidoden- 
drons  and  the  Sigillaria. 

The  Lepidodendron,  or  "  scale  tree,"  was  a  gigantic  club  moss 
fifty  and  seventy-five  feet  high,  spreading  toward  the  top  into 
stout  branches,  at  whose  ends  were  borne  cone-shaped  spore 
cases.  The  younger  parts  of  the  tree  were  clothed  with  stiff 
needle-shaped  leaves,  but  elsewhere  the  trunk  and  branches 
were  marked  with  scalelike  scars,  left  by  the  fallen  leaves,  and 
arranged  in  spiral  rows. 

The  Sigillaria,  or  "  seal  tree,"  was  similar  to  the  Lepidoden- 
dron, but  its  fluted  trunk  divided  into  even  fewer  branches,  and 
was  dotted  with  vertical  rows  of  leaf  scars,  like  the  impressions 
of  a  seal. 


THE  CARBONIFEROUS  363 

Both  Lepidodendron  and  Sigillaria  were  anchored  by  means 
of  great  cablelike  underground  stems,  which  ran  to  long  dis- 
tances through  the  marshy  ground.  The  trunks  of  both  trees 
had  a  thick  woody  rind,  inclosing  loose  cellular  tissue  and  a 
pith.  Their  hollow  stumps,  filled  with  sand  and  mud,  are  com- 
mon in  the  Coal  Measures,  and  in  them  one  sometimes  finds 
leaves  and  stems,  land  shells,  and  the  bones  of  little  reptiles  of 
the  time  which  made  their  home  there. 

It  is  important  to  note  that  some  of  these  gigantic  lycopods, 
which  are  classed  with  the  cryptogams,  or  flowerless  plants,  had 
pith  and  medullary  rays  dividing  their  cylinders  into  woody 
wedges.  These  characters  connect  them  with  the  phanerogams, 
or  flowering  plants.  Like  so  many  of  the  organisms  of  the 
remote  past,  they  were  connecting  types  from  which  groups 
now  widely  separated  have  diverged. 

Gymnosperms,  akin  to  the  cycads,  were  also  present  in  the 
Carboniferous  forests.  Such  were  the  different  species  of  Cor- 
daites,  trees  pyramidal  in  shape,  with  strap-shaped  leaves  and 
nutlike  fruit.  Other  gymnosperms  were  related  to  the  yews, 
and  it  was  by  these  that  many  of  the  fossil  nuts  found  in  the 
Coal  Measures  were  borne.  It  is  thought  by  some  that  the 
gymnosperms  had  their  station  on  the  drier  plains  and  higher 
lands. 

The  Carboniferous  jungles  extended  over  parts  of  Europe 
and  of  Asia,  as  well  as  eastern  North  America,  and  reached 
from  the  equator  to  within  nine  degrees  of  the  north  pole. 
Even  hi  these  widely  separated  regions  the  genera  and  species 
of  coal  plants  are  close  akin  and  often  identical. 

Invertebrates.  Among  the  echinoderms,  crinoids  are  now 
exceedingly  abundant,  sea  urchins  are  more  plentiful,  and  sea 
cucumbers  are  found  now  for  the  first  time.  Trilobites  are 
rapidly  declining,  and  pass  away  forever  with  the  close  of  the 
period.  Eurypterids  are  common ;  stinging  scorpions  are  abun- 
dant ;  and  here  occur  the  first-known  spiders. 


364 


THE   ELEMENTS  OF   GEOLOGY 


We  have  seen  that  the  arthropods  were  the  first  of  all 
animals  to  conquer  the  realm  of  the  air,  the  earliest  insects 
appearing  in  the  Ordovician.  Insects  had  now  become  exceed- 
ingly abundant,  and  the  Carboniferous  forests  swarmed  with 
the  ancestral  types  of  dragon  flies,  —  some  with  a  spread  of  wing 
of  more  than  tw^o  feet, —  May  flies,  crickets,  and  locusts.  Cock- 
roaches infested  the  swamps,  and  one  hundred  and  thirty-three 

species  of  this  ancient  order  have 
been  discovered  in  the  Carbon- 
iferous of  North  America.  The 
higher  flower-loving  insects  are 
still  absent;  the  reign  of  the 


FIG.  309.    Carboniferous  Brachiopods 

A,  Productus;  B,  Spirifer,  the  right-hand  figure  showing  the  interior  with  the 
calcareous  spires  for  the  support  of  the  arms 

flowering  plants  has  not  yet  begun.  The  Paleozoic  insects  were 
generalized  types  connecting  the  present  orders.  Their  fore 
wings  were  still  membranous  and  delicately  veined,  and  used 
in  flying;  they  had  not  yet  become  thick,  and  useful  only  as 
wing  covers,  as  in  many  of  their  descendants. 

Fishes  still  held  to  the  Devonian  types,  with  the  exception 
that  the  strange  ostracoderms  now  had  perished. 

Amphibians.  Footprints  of  amphibians  are  found  in  the 
Devonian.  The  earliest  Carboniferous  amphibians  were  small, 
newtlike  creatures  possessing  not  only  the  typical  amphibian 
double  breathing  system  of  gills  and  lungs  but  also  a  double  loco- 
motive apparatus  of  short,  weak  legs  for  crawling  on  land  and  a 


THE  CARBONIFEROUS 


365 


tail  for  propulsion  in  the  water.    They  branched  into  a  variety 
of  types, — some  large  and  crocodilian,  some  with  well-developed 


FIG.  310.  A  Carboniferous  Dragon  Fly 
One  tenth  natural  size 

legs  for  running,  some  with  large  heads  like  giant  tadpoles, 
and  some  eel-like  and  limbless. 

The  earliest  amphibians  differ  from  those  of  to-day  in  a 
number  of  respects.  They  were  connecting 
types  Unking  together  fishes,  from  which 
they  were  descended,  with  reptiles,  of  which 
they  were  the  ancestors.  They  retained  the 
evidence  of  their  close  relationship  with  the 
Devonian  fishes  in  their  cold  blood,  their 
gills  and  aquatic  habit  during  their  larval 
stage,  their  teeth  with  dentine  infolded  like 
those  of  the  Devonian  ganoids  but  still 
more  intricately,  and  their  biconcave  ver- 
tebrae which  never  completely  ossified. 
These,  the  highest  vertebrates  of  the  time, 
had  not  yet  advanced  beyond  the  embry- 
onic stage  of  the  more  or  less  cartilagi- 
nous skeleton  and  the  persistent  notochord. 


FIG.  311.    A  Carbon- 
iferous Amphibian 


366 


THE  ELEMENTS  OF  GEOLOGY 


On  the  other  hand,  the  skull  of  the  Carboniferous  amphibians 
was  made  of  close-set  bony  plates,  like  the  skull  of  the  reptile, 

rather  than  like  that  of  the  frog, 
with  its  open  spaces  (Figs.  313 
and  314).  Unlike  modern  amphib- 
ians, with  their  slimy  skin,  the 
Carboniferous  amphibians  wore  an 
armor  of  bony  scales  over  the 
ventral  surface  and  sometimes  over 
the  back  as  well. 


-P 


FIG.  312.   Transverse  Section  of 


The  footprint  which  records  the  ap- 
pearance of  the  first  land  vertebrate  in 
Devonian  times  shows  a  foot  with  two 
Segment  of  Tooth  of  Ancient  digits  and  a  rudimentary  budding  third. 
Amphibian  The  Carboniferous  amphibians  had  five- 

toed  feet,  the  primitive  type  of  foot  from  which  their  descendants  of 
higher  orders,  with  a  smaller  number  of  digits,  have  diverged. 

The  Carboniferous  was  the  age  of  lycopods  and  amphibians, 
as  the  Devonian  had  been  the  age  of  rhizocarps  and  fishes. 


FIG.  313.   Skull  of  a  Permian  Amphibian 
from  Texas 


FIG.  314.    Skull  of  a 
Frog 


Life  of  the  Permian.    The  close  of  the  Paleozoic  was,  as  we 
have  seen,  a  time  of  marked  physical  changes.    The  upridging 
of  the  Appalachians  had  begun  and  a  wide  continental  uplift  — 
proved  by  the  absence  of  Permian  deposits  over  large  areas 
where  sedimentation  had  gone  on  before  —  opened  new  lands 


THE   CARBONIFEROUS  367 

for  settlement  to  hordes  of  air-breathing  animals.  Changes  of 
climate  compelled  extensive  migrations,  and  the  fauna  of  differ- 
ent regions  were  thus  brought  into  conflict.  The  Permian  was 
a  time  of  pronounced  changes  in  plant  and  animal  life,  and  a 
transitional  period  between  two  great  eras.  The  somber  forests 
of  the  earlier  Carboniferous,  with  their  gigantic  club  mosses, 
were  now  replaced  by  forests  of  cycads,  tree  ferns,  and  conifers. 
Even  in  the  lower  Permian  the  Lepidodendron  and  Sigillaria 
were  very  rare,  and  before  the  end  of  the  epoch  they  and  the 
Calamites  also  had  become  extinct.  Gradually  the  antique 
types  of  the  Paleozoic  fauna  died  out,  and  in  the  Permian 
rocks  are  found  the  last  survivors  of  the  cystoid,  the  trilobite, 
and  the  eurypterid,  and  of  many  long-lived  families  of  brach- 
iopods,  mollusks,  and  other  invertebrates.  The  venerable  Or- 
thoceras  and  the  Goniatite  linger  on  through  the  epoch  and  into 
the  first  period  of  the  succeeding  era.  Forerunners  of  the  great 
ammonite  family  of  cephalopod  mollusks  now  appear.  The 
antique  forms  of  the  earlier  Carboniferous  amphibians  continue, 
but  with  many  new  genera  and  a  marked  increase  in  size. 

A  long  forward  step  had  now  been  taken  in  the  evolution 
of  the  vertebrates.  A  new  and  higher  type,  the  reptiles,  had 
appeared,  and  in  such  numbers  and  variety  are  they  found  in 
the  Permian  strata  that  their  advent  may  well  have  occurred  in 
a  still  earlier  epoch.  It  will  be  most  convenient  to  describe  the 
Permian  reptiles  along  with  their  descendants  of  the  Mesozoic. 


CHAPTEE  XX 
THE  MESOZOIC 

With  the  close  of  the  Permian  the  world  of  animal  and 
vegetable  life  had  so  changed  that  the  line  is  drawn  here  which 
marks  the  end  of  the  old  order  and  the  beginning  of  the  new  and 
separates  the  Paleozoic  from  the  succeeding  era,  —  the  Mes- 
ozoic,  the  Middle  Age  of  geological  history.  Although  the 
Mesozoic  era  is  shorter  than  the  Paleozoic,  as  measured  by  the 
thickness  of  their  strata,  yet  its  duration  must  be  reckoned  in 
millions  of  years.  Its  predominant  life  features  are  the  culmi- 
nation and  the  beginning  of  the  decline  of  reptiles,  amphibians, 
cephalopod  mollusks,  and  cycads,  and  the  advent  of  marsupial 
mammals,  birds,  teleost  fishes,  and  angiospermous  plants.  The 
leading  events  of  the  long  ages  of  the  era  we  can  sketch  only 
in  the  most  summary  way. 

The  Mesozoic  comprises  three  systems,  —  the  Triassic,  named 
from  its  threefold  division  in  Germany ;  the  Jurassic,  which  is 
well  displayed  in  the  Jura  Mountains  ;  and  the  Cretaceous,  which 
contains  the  extensive  chalk  (Latin,  creta)  deposits  of  Europe. 

In  eastern  North  America  the  Mesozoic  rocks  are  much  less  impor- 
tant than  the  Paleozoic,  for  much  of  this  portion  of  the  continent  was 
land  during  the  Mesozoic  era,  and  the  area  of  the  Mesozoic  rocks  is 
small.  In  western  North  America,  on  the  other  hand,  the  strata  of  the 
Mesozoic  —  and  of  the  Cenozoic  also  —  are  widely  spread.  The  Paleo- 
zoic rocks  are  buried  quite  generally  from  view  except  where  the  moun- 
tain makings  and  continental  uplifts  of  the  Mesozoic  and  Cenozoic 
have  allowed  profound  erosion  to  bring  them  to  light,  as  in  deep  can- 
yons and  about  mountain  axes.  The  record  of  many  of  the  most  impor- 
tant events  in  the  development  of  the  continent  during  the  Mesozoic 
and  Cenozoic  eras  is  found  in  the  rocks  of  our  western  states. 

308 


THE  MESOZOIC  369 

THE  TRIASSIC  AND  JURASSIC 

Eastern  North  America.  The  sedimentary  record  interrupted 
by  the  Appalachian  deformation  was  not  renewed  in  eastern 
North  America  until  late  in  the  Triassic.  Hence  during  this 
long  interval  the  land  stood  high,  the  coast  was  farther  out  than 
now,  and  over  our  Atlantic  states  geological  time  was  recorded 
chiefly  in  erosion  forms  of  hill  and  plain  which  have  long  since 
vanished.  The  area  of  the  later  Triassic  rocks  of  this  region, 
which  take  up  again  the  geological  record,  is  seen  in  the  map 
of  Figure  260.  They  lie  on  the  upturned  and  eroded  edges  of 
the  older  rocks  and  occupy  long  troughs  running  for  the  most 
part  parallel  to  the  Atlantic  coast.  Evidently  subsidence  was  in 
progress  where  these  rocks  were  deposited.  The  eastern  border 
of  Appalachia  was  now  depressed.  The  oldland  was  warping, 
and  long  belts  of  country  lying  parallel  to  the  shore  subsided, 
forming  troughs  in  which  thousands  of  feet  of  sediment  now 
gathered. 

These  Triassic  rocks,  which  are  chiefly  sandstones,  hold  no 
marine  fossils,  and  hence  were  not  laid  in  open  arms  of  the  sea. 
But  their  layers  are  often  ripple-marked,  and  contain  many 
tracks  of  reptiles,  imprints  of  raindrops,  and  some  fossil  wood, 
while  an  occasional  bed  of  shale  is  filled  with  the  remains  of 
fishes.  We  may  conceive,  then,  of  the  Connecticut  valley  and 
the  larger  trough  to  the  southwest  as  basins  gradually  sinking 
at  a  rate  perhaps  no  faster  than  that  of  the  New  Jersey  coast 
to-day,  and  as  gradually  aggraded  by  streams  from  the  neigh- 
boring uplands.  Their  broad,  sandy  flats  were  overflowed  by 
wandering  streams,  and  when  subsidence  gained  on  deposition 
shallow  lakes  overspread  the  alluvial  plains.  Perhaps  now  and 
then  the  basins  became  long,  brackish  estuaries,  whose  low  shores 
were  swept  by  the  incoming  tide  and  were  in  turn  left  bare  at 
its  retreat  to  receive  the  rain  prints  of  passing  showers  and  the 
tracks  of  the  troops  of  reptiles  which  inhabited  these  valleys. 


370         THE  ELEMENTS  OF  GEOLOGY 

The  Triassic  rocks  are  mainly  red  sandstones,  —  often  feldspathic, 
or  arkose,  with  some  conglomerates  and  shales.  Considering  the  large 
amount  of  feldspathic  material  in  these  rocks,  do  you  infer  that  they 
were  derived  from  the  adjacent  crystalline  and  metamorphic  rocks  of 
the  oldland  of  Appalachia,  or  from  the  sedimentary  Paleozoic  rocks 
which  had  been  folded  into  mountains  during  the  Appalachian  defor- 
mation ?  If  from  the  former,  was  the  drainage  of  the  northern  Appa- 
lachian mountain  region  then,  as  now,  eastward  and  southeastward 
toward  the  Atlantic  ?  The  Triassic  sandstones  are  voluminous,  measur- 
ing at  least  a  mile  in  thickness,  and  are  largely  of  coarse  waste.  What 
do  you  infer  as  to  the  height  of  the  lands  from  which  the  waste  was 
shed,  or  the  direction  of  the  oscillation  which  they  were  then  under- 
going ?  In  the  southern  basins,  as  about  Richmond,  Virginia,  are  valu- 
able beds  of  coal ;  what  was  the  physical  geography  of  these  areas  when 
the  coal  was  being  formed? 

Interbedded  with  the  Triassic  sandstones  are  contempora- 
neous lava  beds  which  were  fed  from  dikes.  Volcanic  action, 


FIG.  315.   Section  of  Triassic  Sandstones  of  the  Connecticut  Valley 
ss,  sandstones ;  II,  lava  sheets ;  cc,  crystalline  igneous  and  metamorphic  rocks 

which  had  been  remarkably  absent  in  eastern  North  America 
during  Paleozoic  times,  was  well-marked  in  connection  with 
the  warping  now  in  progress.  Thick  intrusive'  sheets  have  also 
been  driven  in  among  the  strata,  as,  for  example,  the 'sheet  of 
the  Palisades  of  the  Hudson,  described  on  page  269. 

The  present  condition  of  the  Triassic  sandstones  of  the  Connecticut 
valley  is  seen  in  Figure  315.  Were  the  beds  laid  in  their  present  atti- 
tude ?  What  was  the  nature  of  the  deformation  which  they  have  suf- 
fered ?  When  did  the  intrusion  of  lava  sheets  take  place  relative  to  the 
deformation  ?  What  effect  have  these  sheets  on  the  present  topography, 
and  why  ?  Assuming  that  the  Triassic  deformation  went  on  more  rapidly 
than  denudation,  what  was  its  effect  on  the  topography  of  the  time  ?  Are 
there  any  of  its  results  remaining  in  the  topography  of  to-day  ?  Do  the 


THE  MESOZOIC  371 

sic  areas  now  stand  higher  or  lower  than  the  surrounding  country, 
id  why  ?  How  do  the  Triassic  sandstones  and  shales  compare  in  hard- 
ness with  the  igneous  and  metamorphic  rocks  about  them  ?  The  Jurassic 
strata  are  wanting  over  the  Triassic  areas  and  over  all  of  eastern  North 
America.  Was  this  region  laud  or  sea,  an  area  of  erosion  or  sedimenta- 
tion, during  the  Jurassic  period  ?  In  New  Jersey,  Pennsylvania,  and  far- 
ther southwest  the  lowest  strata  of  the  next  period,  the  Cretaceous,  rest 
on  the  eroded  edges  of  the  earlier  rocks.  The  surface  on  which  they  lie 
is  worn  so  even  that  we  must  believe  that  at  the  opening  of  the  Creta- 
ceous the  oldland  of  Appalachia,  including  the  Triassic  areas,  had  been 
baseleveled  at  least  near  the  coast.  When,  therefore,  did  the  deformation 
of  the  Triassic  rocks  occur? 

Western  North  America.  Triassic  strata  infolded  in  the  Sierra 
Nevada  Mountains  carry  marine  fossils  and  reach  a  thickness  of 
nearly  five  thousand  feet.  California  was  then  under  water,  and 
the  site  of  the  Sierra  was  a  subsiding  trough  slowly  filling  with 
waste  from  the  Great  Basin  land  to  the  east. 

Over  a  long  belt  which  reaches  from  Wyoming  across  Colorado  into 
New  Mexico  no  Triassic  sediments  are  found,  nor  is  there  any  evidence 
that  they  were  ever  present ;  hence  this  area  was  high  land  suffering 
erosion  during  the  Triassic.  On  each  side  of  it,  in  eastern  Colorado 
and  about  the  Black  Hills,  in  western  Texas,  in  Utah,  over  the  site  of 
the  Wasatch  Mountains,  and  southward  into  Arizona  over  the  plateaus 
trenched  by  the  Colorado  River,  are  large  areas  of  Triassic  rocks,  sand- 
stones chiefly,  with  some  rock  salt  and  gypsum.  Fossils  are  very  rare 
and  none  of  them  marine.  Here,  then,  lay  broad  shallow  lakes  often 
salt,  and  warped  basins,  in  which  the  waste  of  the  adjacent  uplands 
gathered.  To  this  system  belong  the  sandstones  of  the  Garden  of  the 
Gods  in  Colorado,  which  later  earth  movements  have  upturned  with 
the  uplifted  mountain  flanks. 

The  Jurassic  was  marked  with  varied  oscillations  and  wide 
changes  in  the  outline  of  sea  and  land. 

Jurassic  shales  of  immense  thickness  —  now  metamorphosed 
into  slates  —  are  found  infolded  into  the  Sierra  Nevada  Moun- 
tains. Hence  during  Jurassic  times  the  Sierra  trough  continued 


872  THE  ELEMENTS   OF   GEOLOGY 

to  subside,  and  enormous  deposits  of  mud  were  washed  into  it 
from  the  land  lying  to  the  east.  Contemporaneous  lava  flows 
interbedded  with  the  strata  show  that  volcanic  action  accom- 
panied the  downwarp,  and  that  molten  rock  was  driven  upward 
through  fissures  in  the  crust  and  outspread  over  the  sea  floor  in 
sheets  of  lava. 

The  Sierra  deformation.  Ever  since  the  middle  of  the  Silu- 
rian, the  Sierra  trough  had  been  sinking,  though  110  doubt 
with  halts  and  interruptions,  until  it  contained  nearly  twenty- 
five  thousand  feet  of  sediment.  At  the  close  of  the  Jurassic  it 
yielded  to  lateral  pressure  and  the  vast  pile  of  strata  was  crum- 
pled and  upheaved  into  towering  mountains.  The  Mesozoic 
muds  were  hardened  and  squeezed  into  slates.  The  rocks  were 
wrenched  and  broken,  and  underground  waters  began  the  work 
of  filling  their  fissures  with  gold-bearing  quartz,  which  was  yet  to 
wait  millions  of  years  before  the  arrival  of  man  to  mine  it.  Im- 
mense bodies  of  molten  rock  were  intruded  into  the  crust  as  it  suf-  . 
fered  deformation,  and  these  appear  in  the  large  areas  of  granite 
which  the  later  denudation  of  the  range  has  brought  to  light. 

The  same  movements  probably  uplifted  the  rocks  of  the 
Coast  Eange  in  a  chain  of  islands.  The  whole  western  part  of 
the  continent  was  raised  and  its  seas  and  lakes  were  for  the 
most  part  drained  away. 

The  British  Isles.  The  Triassic  strata  of  the  British  Isles  are  conti- 
nental, and  include  breccia  beds  of  cemented  talus,  deposits  of  salt  and 
gypsum,  and  sandstones  whose  rounded  and  polished  grains  are  those 
of  the  wind-blown  sands  of  deserts.  In  Triassic  times  the  British  Isles 
were  part  of  a  desert  extending  over  much  of  northwestern  Europe. 

THE  CRETACEOUS 

The  third  great  system  of  the  Mesozoic  includes  many  forma- 
tions, marine  and  continental,  which  record  a  long  and  compli- 
cated history  marked  by  great  oscillations  of  the  crust  and  wide 
changes  in  the  outlines  of  sea  and  land. 


THE  MESOZOIC  373 

Early  Cretaceous.  In  eastern  North  America  the  lowest  Cretaceous 
series  comprises  fresh-water  formations  which  are  traced  from  Nantucket 
across  Martha's  Vineyard  and  Long  Island,  and  through  New  Jersey 
southward  into  Georgia.  They  rest  unconformably  on  the  Triassic 
sandstones  and  the  older  rocks  of  the  region.  The  Atlantic  shore  line 
was  still  farther  out  than  now  in  the  northern  states.  Again,  as  during 
the  Triassic,  a  warping  of  the  crust  formed  a  long  trough  parallel  to  the 
coast  and  to  the  Appalachian  ridges,  but  cut  off  from  the  sea ;  and 
here  the  continental  deposits  of  the  early  Cretaceous  were  laid. 

Along  the  Gulf  of  Mexico  the  same  series  was  deposited  under  like 
conditions  over  the  area  known  as  the  Mississippi  embayment,  reaching 
from  Georgia  northwestward  into  Tennessee  and  thence  across  into 
Arkansas  and  southward  into  Texas. 

In  the  Southwest  the  subsidence  continued  until  the  transgressing 
sea  covered  most  of  Mexico  and  Texas  and  extended  a  gulf  northward 
into  Kansas.  In  its  warm  and  quiet  waters  limestones  accumulated  to 
a  depth  of  from  one  thousand  to  five  thousand  feet  in  Texas,  and  of 
more  than  ten  thousand  feet  in  Mexico.  Meanwhile  the  lowlands,  where 
the  Great  Plains  are  now,  received  continental  deposits ;  coal  swamps 
stretched  from  western  Montana  into  British  Columbia. 

The  Middle  Cretaceous.  This  was  a  land  epoch.  The  early  Cretaceous 
sea  retired  from  Texas  and  Mexico,  for  its  sediments  are  overlain 
unconformably  by  formations  of  the  Upper  Cretaceous.  So  long  was 
the  time  gap  between  the  two  series  that  no  species  found  in  the  one 
occurs  in  the  other. 

The  Upper  Cretaceous.  There  now  began  one  of  the  most 
remarkable  events  in  all  geological  history,  —  the  great  Creta- 
ceous subsidence.  Its  earlier  warpings  were  recorded  in  conti- 
nental deposits,  —  wide  sheets  of  sandstone,  shale,  and  some 
coal,  —  which  were  spread  from  Texas  to  British  Columbia. 
These  continental  deposits  are  overlain  by  a  succession  of  marine 
formations  whose  vast  area  is  shown  on  the  map,  Figure  260.  We 
may  infer  that  as  the  depression  of  the  continent  continued  the  sea 
came  in  far  and  wide  over  the  coast  lands  and  the  plains  worn 
low  during  the  previous  epochs.  Upper  Cretaceous  formations 
show  that  south  of  New  England  the  waters  of  the  Atlantic 


374 


THE  ELEMENTS  OF   GEOLOGY 


somewhat  overlapped  the  crystalline  rocks  of  the  Piedmont  Belt 
and  spread  their  waste  over  the  submerged  coastal  plain.  The 
Gulf  of  Mexico  again  covered  the  Mississippi  embayment,  reach- 


FIG.  316.   Hypothetical  Map  of  Upper  Cretaceous  Epicontinental  Seas 
Shaded  areas,  probable  seas ;  broken  lines,  approximate  shore  lines 

ing  as  far  north  as  southern  Illinois,  and  extended  over  Texas. 

A  mediterranean  sea  now  stretched  from  the  Gulf  to  the  arctic 

regions  and  from  central 
Iowa  to  the  eastern  shore  of 
the  Great  Basin  land  at  about 
the  longitude  of  Salt  Lake 
City,  the  Colorado  Mountains 
rising  from  it  in  a  chain  of 
islands.  Along  with  minor 
oscillations  there  were  laid 
in  the  interior  sea  various 
formations  of  sandstones, 
FIG.  317.  Foraminifera  from  Creta-  shales,  and  limestones,  and 
ceous  Chalk,  Iowa.  Magnified  from  Kansas  to  South  Dakota 

beds  of  white  chalk  show  that  the  clear,  warm  waters  swarmed 

at  times  with  foraminiferal  life  whose  disintegrating  microscopic 

shells  accumulated  in  this  rare  deposit. 


THE  MESOZOIC  375 

At  this  epoch  a  wide  sea,  interrupted  by  various  islands,  stretched 
across  Eurasia  from  Wales  and  western  Spain  to  China,  and  spread 
southward  over  much  of  the  Sahara.  To  the  west  its  waters  were  clear 
and  on  its  floor  the  crumbled  remains  of  foraminifers  gathered  in 
heavy  accumulations  of  calcareous  ooze,  —  the  white  chalk  of  France 
and  England.  Sea  urchins  were  also  abundant,  and  sponges  contributed 
their  spicules  to  form  nodules  of  flint. 

The  Laramie.  The  closing  stage  of  the  Cretaceous  was  marked  in 
North  America  by  a  slow  uplift  of  the  land.  As  the  interior  sea  gradu- 
ally withdrew,  the  warping  basins  of  its  floor  were  filled  with  waste 
from  the  rising  lands  about  them,  and  over  this  wide  area  there  were 
spread  continental  deposits  in  fresh-water  lakes  like  the  Great  Lakes  of 
the  present,  in  brackish  estuaries,  and  in  river  plains,  while  occasional 
oscillations  now  and  again  let  in  the  sea.  There  were  vast  marshes  in 
which  there  accumulated  the  larger  part  of  the  valuable  coal  seams  of 
the  West.  The  Laramie  is  the  coal-bearing  series  of  the  West,  as  the 
Pennsylvanian  is  of  the  eastern  part  of  our  country. 

The  Rocky  Mountain  deformation.  At  the  close  of  the  Cre- 
taceous we  enter  upon  an  epoch  of  mountain-making  far  more 
extensive  than  any  which  the  continent  had  witnessed.  The 
long  belt  lying  west  of  the  ancient  axes  of  the  Colorado  Islands 
and  east  of  the  Great  Basin  land  had  been  an  area  of  deposition 
for  many  ages,  and  in  its  subsiding  troughs  Paleozoic  and  Meso- 
zoic  sediments  had  gathered  to  the  depth  of  many  thousand 
feet.  And  now  from  Mexico  well-nigh  to  the  Arctic  Ocean  this 
belt  yielded  to  lateral  pressure.  The  Cretaceous  limestones  of 
Mexico  were  folded  into  lofty  mountains.  A  massive  range  was 
upfolded  where  the  Wasatch  Mountains  now  are,  and  various 
ranges  of  the  Kockies  in  Colorado  and  other  states  were  upridged. 
However  slowly  these  deformations  were  effected  they  were  no 
doubt  accompanied  by  world-shaking  earthquakes,  and  it  is 
known  that  volcanic  eruptions  took  place  on  a  magnificent 
scale.  Outflows  of  lava  occurred  along  the  Wasatch,  the  lacco- 
liths of  the  Henry  Mountains  (p.  271)  were  formed,  while  the 
great  masses  of  igneous  rock  which  constitute  the  cores  of  the 


376  THE  ELEMENTS   OF   GEOLOGY 

Spanish  Peaks  (p.  271)  and  other  western  mountains  were  thrust 
up  amid  the  strata.  The  high  plateaus  from  which  many  of 
these  ranges  rise  had  not  yet  been  uplifted,  and  the  bases  of  the 
mountains  probably  stood  near  the  level  of  the  sea. 

North  America  was  now  well-nigh  completed.  The  medi- 
terranean seas  which  so  often  had  occupied  the  heart  of  the 
land  were  done  away  with,  and  the  continent  stretched  unbroken 
from  the  foot  of  the  Sierras  on  the  west  to  the  Fall  "Line  of  the 
Atlantic  coastal  plain  on  the  east. 

The  Mesozoic  peneplain.  The  immense  thickness  of  the  Mes- 
ozoic  formations  conveys  to  our  minds  some  idea  of  the  vast 
length  of  time  involved  in  the  slow  progress  of  its  successive 
ages.  The  same  lesson  is  taught  as  plainly  by  the  amount  of 
denudation  which  the  lands  suffered  during  the  era. 

The  beginning  of  the  Mesozoic  saw  a  system  of  lofty  mountain 
ranges  stretching  from  New  York  into  central  Alabama.  The 
end  of  this  long  era  found  here  a  wide  peneplain  crossed  by 
sluggish  wandering  rivers  and  overlooked  by  detached  hills  as 
yet  unreduced  to  the  general  level.  The  Mesozoic  era  was  long 
enough  for  the  Appalachian  Mountains,  upridged  at  its  begin- 
ning, to  have  been  weathered  and  worn  away  and  carried  grain 
by  grain  to  the  sea.  The  same  plain  extended  over  southern 
New  England.  The  Taconic  range,  uplifted  partially  at  least  at 
the  close  of  the  Ordovician,  and  the  block  mountains  of  the 
Triassic,  together  with  the  pre-Cambrian  mountains  of  ancient 
Appalachia,  had  now  all  been  worn  to  a  common  level  with  the 
Allegheny  ranges.  The  Mesozoic  peneplain  has  been  upwarped 
by  later  crustal  movements  and  has  suffered  profound  erosion,  but 
the  remnants  of  it  which  remain  on  the  upland  of  southern  New 
England  and  the  even  summits  of  the  Allegheny  ridges  suffice 
to  prove  that  it  once  existed.  The  age  of  the  Mesozoic  peneplain 
is  determined  from  the  fact  that  the  lower  Tertiary  sediments 
were  deposited  on  its  even  surface  when  at  the  close  of  the  era 
the  peneplain  was  depressed  along  its  edges  beneath  the  sea. 


THE  MESOZOIC 


377 


LIFE  OF  THE  MESOZOIC   . 

Plant  life  of  the  Triassic  and  Jurassic.  The  Carboniferous 
forests  of  lepidodendrons  and  sigillarids  had  now  vanished 
from  the  earth.  The  uplands  were  clothed  with  conifers,  like 
the  Araucarian  pines  of  South  America  and  Australia.  Dense 
forests  of  tree  ferns  throve  in  moist  regions,  and  canebrakes  of 
horsetails  of  modern  type,  but  with  stems  reaching  four  inches 
in  thickness,  bordered  the  lagoons  and  marshes.  Cycads  were 


FIG.  318.  A  Living  Cycad  of 
Australia 


FIG.  319.  Stem  of  a  Mesozoic 
Cycad 


exceedingly  abundant.  These  gymnosperms,  related  to  the  pines 
and  spruces  in  structure  and  fruiting,  but  palmlike  in  their  foli- 
age, and  uncoiling  their  long  leaves  after  the  manner  of  ferns, 
culminated  in  the  Jurassic.  From  the  view  point  of  the  bota- 
nist the  Mesozoic  is  the  Age  of  Cycads,  and  after  this  era  they 
gradually  decline  to  the  small  number  of  species  now  existing  in 
tropical  latitudes. 

Plant  life  of  the  Cretaceous.  In  the  Lower  Cretaceous  the 
woodlands  continued  of  much  the  same  type  as  during  the 
Jurassic.  The  forerunners  now  appeared  of  the  modem  dicotyls 
(plants  with  two  seed  leaves),  and  in  the  Middle  Cretaceous  the 
monocotyledonous  group  of  palms  came  in.  Palms  are  so  like 
cycads  that  we  may  regard  them  as  the  descendants  of  some 
cycad  type. 


378         THE  ELEMENTS  OF  GEOLOGY 

In  the  Upper  Cretaceous,  cycads  become  rare.  The  highest 
types  of  flowering  plants  gain  a  complete  ascendency,  and 
forests  of  modern  aspect  cover  the  continent  from  the  Gulf  of 
Mexico  to  the  Arctic  Ocean.  Among  the  kinds  of  forest  trees 
whose  remains  are  found  in  the  continental  deposits  of  the 
Cretaceous  are  the  magnolia,  the  myrtle,  the  laurel,  the  fig, 
the  tulip  tree,  the  chestnut,  the  oak,  beech,  elm,  poplar,  wil- 
low, birch,  and  maple.  Forests  of  Eucalyptus  grew  along  the 
coast  of  New  England,  and  palms  on  the  Pacific  shores  of 
British  Columbia.  Sequoias  of  many  varieties  ranged  far  into 
northern  Canada.  In  northern  Greenland  there  were  luxuriant 
forests  of  magnolias,  figs,  and  cycads ;  and  a  similar  flora  has 
been  disinterred  from  the  Cretaceous  rocks  of  Alaska  and  Spitz- 
bergen.  Evidently  the  lands  within  the  Arctic  Circle  enjoyed  a 
warm  and  genial  climate,  as  they  had  done  during  the  Paleozoic. 
Greenland  had  the  temperature  of  Cuba  and  southern  Florida, 
and  the  time  was  yet  far  distant  when  it  was  to  be  wrapped  in 
glacier  ice. 

Invertebrates.  During  the  long  succession  of  the  ages  of  the 
Mesozoic,  with  their  vast  geographical  changes,  there  were  many 

and  great  changes 
in  organisms.  Spe- 
cies were  replaced 
again  and  again  by 
others  better  fitted 
to  the  changing  en- 
vironment. During 
FIG.  320.  A  Jurassic  Long-Tailed  Crustacean 

the  Lower  Creta- 
ceous alone  there  were  no  less  than  six  successive  changes  in 
the  faunas  which  inhabited  the  limestone-making  sea  which 
then  covered  Texas.  We  shall  disregard  these  changes  for  the 
most  part  in  describing  the  life  of  the  era,  and  shall  confine 
our  view  to  some  of  the  most  important  advances  made  in  the 
leading  types. 


THE  MESOZOIC 


379 


Stromatopora  have  disappeared.  Protozoans  and  sponges  are 
exceedingly  abundant,  and  all  contribute  to  the  making  of 
Mesozoic  strata.  Corals 
have  assumed  a  more 
modern  type.  Sea 
urchins  have  become 
plentiful;  crinoids 
abound  until  the  Cre- 
taceous, where  they 
begin  their  decline  to 
their  present  humble 
station. 

Trilobites  and  euryp- 
terids  are  gone.  Ten- 
footed  crustaceans  abound  of  the  primitive  long-tailed  type 
(represented  by  the  lobster  and  the  crayfish),  and  in  the  Jurassic 
there  appears  the  modern  short-tailed  type  represented  by  the 
crabs.  The  latter  type  is  higher  hi  organ- 
ization and  now  far  more  common.  In 
its  embryological  development  it  passes 


FIG.  321.   A  Fossil  Crab 


FIG.  322.   Cretaceous  Mollusks 
A,  Ostrea  (oyster) ;  B,  Exogyra;  C,  Gryphsea 

through  the  long-tailed  stage ;  connecting  links  in  the  Meso- 
zoic  also  indicate  that  the  younger  type  is  the  offshoot  of 
the  older. 


380 


THE  ELEMENTS  OF  GEOLOGY 


Insects  evolve  along  diverse  lines,  giving  rise  to  beetles,  ants, 
bees,  and  flies. 

Brachiopods  have  dwindled  greatly  in  the  number  of  their 
species,  while  mollusks  have  correspondingly  increased.  The 
great  oyster  family  dates  from  here. 

Cephalopods  are  now  to  have  their  day.  The  archaic  Or- 
thoceras  lingers  011  into  the  Triassic  and  becomes  extinct,  but  a 
remarkable  development  is  now  at  hand  for  the  more  highly 
organized  descendants  of  this 
ancient  line.  We  have  noticed 
Hint  in  the  Devonian  the 


FIG.  323.    Ceratites  FIG.  324.    An  Ammonite 

A  portion  of  the  shell  is  removed  to  show 
frilling  of  suture 

sutures  of  some  of  the  chambered  shells  become  angled,  evolv- 
ing the  Goniatite  type  (p.  344).  The  sutures  now  become  lobed 
and  corrugated  in  Ceratites.  The  process  was  carried  still  farther, 
and  the  sutures  were  elaborately  frilled  in  the  great  order  of 
the  Ammonites  (Fig.  324).  It  was  in  the  Jurassic  that  the  Am- 
monites reached  their  height.  No  fossils  are  more  abundant 
or  characteristic  of  their  age.  Great  banks  of  their  shells  formed 
beds  of  limestone  in  warm  seas  the  world  over. 


THE  MESOZOIC 


381 


FIG.  325.  Slab  of  Rock  covered  with  Ammonites,  —  a  Bit  of 
a  Mesozoic  Sea  Bottom 


FIG.  326.   Representative  Species  of  Different  Families 
of  Ammonoids 


382 


THE  ELEMENTS  OF  GEOLOGY 


The  ammonite  'stem  branched  into  a  most  luxuriant  variety 
of  forms.  The  typical  form  was  closely  coiled  like  a  nautilus 
(Fig.  325).  In  others  (Fig.  326)  the  coil  was  more  or  less  open, 
or  even  erected  into  a  spiral.  Some  were  hook-shaped,  and  there 
were  members  of  the  order  in  which  the  shell  was  straight,  and 
yet  retained  all  the  internal  structures  of  its  kind.  At  the  end 
of  the  Mesozoic  the  entire  tribe  of  ammonites  became  extinct. 

The  Belemnite  (Greek,  belemnon,  a  dart)  is  a 
distinctly  higher  type  of  cephalopod  which  ap- 
peared in  the  Triassic,  became  numerous  and 
varied  in  the  Jurassic  and  Cretaceous,  and  died 
out  early  in  the  Tertiary.  Like  the  squids  and 
cuttlefish,  of  which  it  was  the  prototype,  it  had 
an  internal  calcareous  shell  (Fig.  327).  This 
consisted  of  a  chambered  and  siphuncled  cone 
(Fig.  327,  Ph),  whose  point  was  sheathed  in  a 
long  solid  guard  (Fig.  327,  R)  somewhat  like  a 
dart.  The  animal  carried  an  ink  sac,  and  no 
doubt  used  it  as  that  of  the  modern  cuttlefish 
is  used,  —  to  darken  the  water  and  make  easy 
an  escape  from  foes.  Belemnites  have  some- 
times been  sketched  with  fossil  sepia,  or  india 
ink,  from  their  own  ink  sacs.  In  the  belemnites 
and  their  descendants,  the  squids  and  cuttle- 
FIG.  327.  Internal  fish,  the  cephalopods  made  the  radical  change 
'lemnite  from  the  external  to  the  internal  shell.  They 
abandoned  the  defensive  system  of  warfare  and  boldly  took  up 
the  offensive.  No  doubt,  like  their  descendants,  the  belemnites 
were  exceedingly  active  and  voracious  creatures. 

Fishes  and  amphibians.  In  the  Triassic  and  Jurassic,  little 
progress  was  made  among  the  fishes,  and  the  ganoid  was  still 
the  leading  type.  In  the  Cretaceous  the  teleosts,  or  bony 
fishes  (p.  349),  made  their  appearance,  while  ganoids  declined 
toward  their  present  subordinate  place. 


-Ph. 


THE  MESOZOIC  383 

The  amphibians  culminated  in  the  Triassic,  some  being  formi- 
dable creatures  as  large  as  alligators.  They  were  still  of  the 
primitive  Paleozoic  types  (p.  364).  Their  pygmy  descendants 
of  more  modern  types  are  not  found  until  later,  salamanders 
appearing  first  in  the  Cretaceous,  and  frogs  at  the  beginning  of 
the  Cenozoic. 

No  remains  of  amphibians  have  been  discovered  in  the  Jurassic.  Do 
you  infer  from  this  that  there  were  none  in  existence  at  that  time  ? 

Reptiles  of  the  Mesozoic 

The  great  order  of  Keptiles  made  its  advent  in  the  Permian, 
culminated  in  the  Triassic  and  Jurassic,  and  began  to  decline 
in  the  Cretaceous.  The  advance  from  the  amphibian  to  the 
reptile  was  a  long  forward  step  in  the  evolution  of  the  verte- 
brates. In  the  reptile  the  vertebrate  skeleton  now  became  com- 
pletely ossified.  Gills  were  abandoned  and  breathing  was  by 
lungs  alone.  The  development  of  the  individual  from  the  egg 
to  maturity  was  uninterrupted  by  any  metamorphosis,  such  as 
that  of  the  frog  when  it  passes  from  the  tadpole  stage.  Yet  in 
advancing  from  the  amphibian  to  the  reptile  the  evolution  of 
the  vertebrate  was  far  from  finished.  The  cold-blooded,  clumsy 
and  sluggish,  small-brained  and  unintelligent  reptile  is  as  far 
inferior  to  the  higher  mammals,  whose  day  was  still  to  come,  as 
it  is  superior  to  the  amphibian  and  the  fish. 

The  reptiles  of  the  Permian,  the  earliest  known,  were  much 
like  lizards  in  form  of  body.  Constituting  a  transition  type 
between  the  amphibians  on  the  one  hand,  and  both  the  higher 
reptiles  and  the  mammals  on  the  other,  they  retained  the 
archaic  biconcave  vertebrae  of  the  fish  and  in  some  cases  the 
persistent  notochord,  while  some  of  them,  the  theromorphs, 
possessed  characters  allying  them  with  mammals.  In  these  the 
skull  was  remarkably  similar  to  that  of  the  carnivores,  or  flesh- 
eating  mammals,  and  the  teeth,  unlike  the  teeth  of  any  later 


384         THE  ELEMENTS  OF  GEOLOGY 

reptiles,  were  divisible  into  incisors,  canines,  and  molars,  as  are 

the  teeth  of  mammals  (Fig.  328). 

At  the  opening  of  the  Mesozoic  era  reptiles  were  the  most 

highly  organized  and  powerful  of  any  animals  on.  the  earth. 

New  ranges  of  continental  extent  were  opened  to  them,  food 

was  abundant,  the  climate  was  congenial,  and  they  now  branched 

into  very  many  diverse  types 
which  occupied  and  ruled 
all  fields,  —  the  land,  the  air, 
and  the  sea.  The  Mesozoic 
was  the  Age  of  Reptiles. 
The  ancestry  of  surviving 

FIG.  328.  Skull  of  a  Permian  Theromorph  reptilian    types.     We    will 

consider  first  the  evolution  of  the  few  reptilian  types  which 
have  survived  to  the  present. 

Crocodiles,  the  highest  of  existing  reptiles,  are  a  very  ancient 
order,  dating  back  to  the  lower  Jurassic,  and  traceable  to  earlier 
ancestral,  generalized  forms,  from  which  sprang  several  other 
orders  also. 

Turtles  and  tortoises  are  not  found  until  the  early  Jurassic, 
when  they  already  possessed  the  peculiar  characteristics  which 
set  them  off  so  sharply  from  other  reptiles.  They  seem  to  have 
lived  at  first  in  shallow  water  and  in  swamps,  and  it  is  not  until 
after  the  end  of  the  Mesozoic  that  some  of  the  order  became 
adapted  to  life  on  the  land. 

The  largest  of  all  known  turtles,  Archelon,  whose  home  was  the 
great  interior  Cretaceous  sea,  was  fully  a  dozen  feet  in  length  and  must 
have  weighed  at  least  two  tons.  The  skull  alone  is  a  yard  long. 

Lizards  and  snakes  do  not  appear  until  after  the  close  of  the 
Mesozoic,  although  their  ancestral  lines  may  be  followed  back 
into  the  Cretaceous. 

We  will  now  describe  some  of  the  highly  specialized  orders 
peculiar  to  the  Mesozoic. 


THE  MESOZOIC 


385 


Land  reptiles.  The  dinosaurs  (terrible  reptiles)  are  an  ex- 
tremely varied  order  which  were  masters  of  the  land  from 
the  late  Trias  until  the  close  of  the  Mesozoic  era.  Some 
were  far  larger  than  elephants,  some  were  as  small  as  cats ; 
some  walked  on  all  fours,  some  were  bipedal ;  some  fed  on  the 
luxuriant  tropical  foliage,  and  others  on  the  flesh  of  weaker 
reptiles.  They  may  be  classed  in  three  divisions,  —  the  flesh- 
eating  dinosaurs,  the  reptile-footed  dinosaurs,  and  the  leaked 
dinosaurs,  —  the  latter  two  divisions  being  herbivorous. 

The  flesh-eating  dinosaurs  are  the  oldest  known  division  of 
the  order,  and  their  characteristics  are  shown  in  Figure  329. 
As  a  class,  reptiles  are  egg  layers  (oviparous) ;  but  some  of 
the  flesh-eating  dinosaurs  are  known  to  have  been  viviparous, 
i.e.  to  have  brought  forth  their  young  alive.  This  group  was 
the  longest-lived  of  any  of  the  three,  beginning  in  the  Trias 
and  continuing  to  the  close  of  the  Mesozoic  era. 


FIG.  329.    Ceratosaurus 
From'  Animals  of  the  Past.    By  the  Courtesy  of  McClure,  Phillips  &  Co. 

Contrast  the  small  fore  limbs,  used  only  for  grasping,  with  the 
powerful  hind  limbs  on  which  the  animal  stalked  about.  Note  the  sharp 
claws  and  ponderous  tail.  The  ceratosaur  was  fifteen  feet  long,  but  a 


386 


THE  ELEMENTS  OF  GEOLOGY 


very  similar  monster,  the  tyrannosaur, 
reached  a  length  of  forty-seven  feet  and 
a  standing  height  of  twenty  feet.  "In 
speed,  size,  power,  and  ferocity  "  this  rep- 
tile was  "the  most  destructive  life  engine 
which  has  ever  evolved."  Its  swift  rush 
must  have  been  well-nigh  resistless. 

The  reptile-footed  dinosaurs  (Sauro- 
poda)  include  some  of  the  biggest 
brutes  which  ever  trod  the  ground. 
One  of  the  largest,  whose  remains  are 
found  entombed  in  the  Jurassic  rocks 
of  Wyoming  and  Colorado,  is  shown 
in  Figure  330. 

Note  the  five  digits  on  the  .hind  feet, 
the  quadrupedal  gait,  the  enormous  stretch 
of  neck  and  tail,  the  small  head  aligned 
with  the  vertebral  column.  Diplodocus  was 
fully  sixty-five  feet  long  and  must  have 
weighed  about  twenty  tons.  The  thigh 
bones  of  the  Sauropoda  are  the  largest 
bones  which  ever  grew.  That  of  a  genus 
allied  to  the  Diplodocus  measures  six  feet 
and  eight  inches,  and  the  total  length  of 
the  animal  must  have  been  not  far  from 
eighty  feet,  the  largest  land  animal  known. 

The  Sauropoda  became  extinct 
when  their  haunts  along  the  rivers 
and  lakes  of  the  western  plains  of 
Jurassic  times  were  invaded  by  the 
Cretaceous  interior  sea. 

The  leaked  dinosaurs  (Predentata) 
were  distinguished  by  a  beak  sheathed 
•with  horn  carried  in  front  of  the  tooth- 
set  jaw,  and  used,  we  may  imagine,  in 


THE  MESOZOIC 


387 


stripping  the  leaves  and  twigs  of  trees  and  shrubs.    We  may 
notice  only  two  of  the  most  interesting  types. 

Stegosaurus  (plated  reptile)  takes  its  name  from  the  double  row  of 
bony  plates  arranged  along  its  back.  The  powerful  tail  was  armed 
with  long  spines,  and  the  thick  skin  was  defended  with  irregular  bits 
of  bone  implanted  in  it.  The  brain  of  the  stegosaur  was  smaller  than 
that  of  any  land  vertebrate,  while  in  the  sacrum  the  nerve  canal  was 
enlarged  to  ten  times  the  capacity  of  the  brain  cavity  of  the  skull. 
Despite  their  feeble  wits,  this  well-armored  family  lived  on  through 


FIG.  331.  Stegosaurus 

millions  of  years  which  intervened  between  their  appearance,  at  the 
opening  of  the  Jurassic,  and  the  close  of  the  Cretaceous,  when  they 
became  extinct. 

A  less  stupid  brute  than  the  stegosaur  was  Triceratops,  the  dinosaur 
of  the  three  horns,  —  one  horn  carried  on  the  nose,  and  a  massive  pair 
set  over  the  eyes  (Fig.  332).  Note  the  enormous  wedge-shaped  skull, 
with  its  sharp  beak,  and  the  hood  behind  resembling  a  fireman's  helmet. 
Triceratops  was  fully  twenty-five  feet  long,  and  of  twice  the  bulk  of  an 
elephant.  The  family  appeared  in  the  Upper  Cretaceous  and  became 
extinct  at  its  close.  Their  bones  are  found  buried  in  the  fresh-water 
deposits  of  the  time  from  Colorado  to  Montana  and  eastward  to  the 
Dakotas. 


THE  MESOZOIC 


389 


Marine  reptiles.  In  the  ocean,  reptiles  occupied  the  place 
now  held  by  the  aquatic  mammals,  such  as  whales  and  dol- 
phins, and  their  form  and  structure  were  similarly  modified  to 
suit  their  environment.  In  the  Ichthyosaurus  (fish  reptile),  for 


FIG.  333.   Ichthyosaurus 

example,  the  body  was  fishlike  in  form,  with  short  neck  and 
large,  pointed  head  (Fig.  333). 

A  powerful  tail,  whose  flukes  were  set  vertical,  and  the  lower  one  of 
which  was  vertebrated,  served  as  propeller,  while  a  large  dorsal  fin  was 
developed  as  a  cutwater.  The  primitive  biconcave  vertebrae  of  the  fish 


FIG.  334.   Plesiosaurus 

and  of  the  early  land  vertebrates  were  retained,  and  the  limbs  degen- 
erated into  short  paddles.  The  skin  of  the  ichthyosaur  was  smooth 
like  that  of  a  whale,  and  its  food  was  largely  fish  and  cephalopods,  as 
the  fossil  contents  of  its  stomach  prove. 

These  sea  monsters  disported  along  the  Pacific  shore  over 
northern  California  in  Triassic  times,  and  the  bones  of  immense 
members  of  the  family  occur  in  the  Jurassic  strata  of  Wyoming. 


390 


THE  ELEMENTS   OF   GEOLOGY 


Like  whales  and  seals,  the  ichthyosaurs  were  descended  from 

land  vertebrates  which  had  become  adapted  to  a  marine  habitat. 

Plesiosaurs  were    another   order   which   ranged    throughout 

the  Mesozoic.    Descended  from  small  amphibious  animals,  they 

later  included  great 
marine  reptiles, 
characterized  in  the 
typical  genus  by 
long  neck,  snakelike 
head,  and  immense 
paddles.  They  swam 
in  the  Cretaceous 
interior  sea  of  west- 


ern North  America. 
Mosasaurs  belong 
to  the    same    order 
as  do  snakes  and 


ancestral  line  of 
land  reptiles.  These 
snakelike  creatures 

—  which  measured 
as    much    as   forty- 
five   feet   in   length 

—  abounded  in  the 
Cretaceous    seas. 

They  had   large   conical   teeth,   and  their  limbs   had    become 
stout  paddles. 


FIG.  335.    Restoration  of  a  Mosasaur 

From  Animals  of  the  Past.    By  the  courtesy  of 
McClui-e,  Phillips  &  Co. 


The  lower  jaw  of  the  mosasaur  was  jointed ;  the  quadrate  bone, 
which  in  all  reptiles  connects  the  bone  of  the  lower  jaw  with  the  skull, 
was  movable,  and  as  in  snakes  the  lower  jaw  could  be  used  in  thrust- 
ing prey  down  the  throat.  The  family  became  extinct  at  the  end  of  the 
Mesozoic,  and  left  no  descendants.  One  may  imitate  the  movement  of 


THE  MESOZOIC 


391 


the  lower  jaw  of  the  mosasaur  by  extending  the  arms,  clasping  the 
hands,  and  bending  the  elbows. 

Flying  reptiles.    The  atmosphere,  which  had  hitherto  been 
tenanted  only  by  insects,  was  first  conquered  by  the  verte- 

brates  in  the 
Meso'zoic.  Pter- 
osaurs, winged 
reptiles,  whose 
whole  organism 
was  adapted  for 
flight  through  the 
air,  appeared  in 
the  Jurassic  and  passed  off  the  stage  of  existence  before  the 
end  of  the  Cretaceous.  The  bones  were  hollow,  as  are  those  of 
birds.  The  sternum,  or  breastbone,  was  given  a  keel  for  the 
attachment  of  the  wing  muscles.  The  fifth  finger,  prodigiously 


FIG.  336.   Restoration  of  a  Pterosaur 


FIG.  337.   Skeletons  of  the  Pterosaur  Oruithostoma,  A, 
and  of  the  Condor,  B 

After  Lucas 

lengthened,  was  turned  backward  to  support  a  membrane  which 
was  attached  to  the  body  and  extended  to  the  base  of  the  tail 
The  other  fingers  were  free,  and  armed  with  sharp  and  delicate 
claws,  as  shown  in  Figures  336  and  337. 


392 


THE  ELEMENTS  OF  GEOLOGY 


n 


These  "  dragons  of  the  air "  varied  greatly  in  size ;  some  were  as 
small  as  sparrows,  while  others  surpassed  in  stretch  of  wing  the  largest 
birds  of  the  present  day.  They  may  be  divided  into  two  groups.  The 
earliest  group  comprises  genera  with  jaws  set  with  teeth,  and  with  long 

tails  sometimes  provided 
with  a  rudderlike  ex- 
pansion at  .the  end.  In 
their  successors  of  the 
later  group  the  tail  had 
become  short,  and  in 
some  of  the  genera  the 
teeth  had  disappeared. 
Among  the  latest  of  the 
flying  reptiles  was  Orni- 
thosloma  (bird  beak),  the 
largest  creature  which 
ever  flew,  and  whose  re- 
mains are  imbedded  in 
the  offshore  deposits  of 
the  Cretaceous  sea  which 
held  sway  over  our  west- 
ern plains.  Ornithosto- 
ma's  spread  of  wings 
was  twenty  feet.  Its 
bones  were  a  marvel  of 
lightness,  the  entire 
skeleton,  even  in  its  pet- 
rified condition,  not 
weighing  more  than  five 
or  six  pounds.  The 
sharp  beak,  a  yard  long, 
was  toothless  and  bird- 
like,  as  its  name  sug- 


IV  H 


Ifl 


FIG.  338.   Archseopteryx 


gests. 


Birds.  The  earliest-known  birds  are  found  in  the  Jurassic, 
and  during  the  remainder  of  the  Mesozoic  they  contended  with 
the  flying  reptiles  for  the  empire  of  the  air.  The  first  feathered 


THE  MESOZOIC 

creatures  were  very  different  from  the  birds  of  to-day.  Their 
characteristics  prove  them  an  offshoot  of  the  dinosaur  line 
of  reptiles.  Archceopteryx  (ancient  bird)  (Fig.  338)  exhibits  a 
strange  mingling  of  bird  and  reptile.  like  birds,  it  was  fledged 
with  perfect  feathers,  at  least  on  wings  and  tail,  but  it  retained 
the  teeth  of  the  reptile,  and  its  long  tail  was  vertebrated,  a  pair 
of  feathers  springing  from  each  joint.  Three  fingers  were  free 
and  clawed.  There  were  quill  feathers  on  the  legs.  This  and 
the  fact  that  young  birds  have  sprouting  quills  on  the  le^ 
from  knee  to  tail  suggest  a  four-winged  ancestor  capable  of 
gliding  leap  from  trees,  whose  feathers  were  developing  from 
reptilian  scales. 

Mammals.  So  far  as  the  entries  upon  the  geological  record 
show,  mammals  made  their  advent  in  a  very  humble  way  dur- 
ing the  Trias.  These  earliest  of  vertebrates  which  suckle  their 
young  were  no  bigger  than  young 
kittens,  and  their  strong  affinities 
with  the  theromorphs  suggest  that 
their  ancestors  are  to  be  found 

among  some   generalized   types   of        FlG-  3^9.  Jawbone  of  a 
,v  j         £         .'-I  Jurassic  Mammal 

that  order  of  reptiles. 

During  the  long  ages  of  the  Mesozoic,  mammals  continued 
small  and  few,  and  were  completely  dominated  by  the  reptiles. 
Their  remains  are  exceedingly  rare,  and  consist  of  minute  scat- 
tered teeth,  —  with  an  occasional  detached  jaw,  —  which  prove 
them  to  have  been  flesh  or  insect  eaters.  In  the  same  way 
their  affinities  are  seen  to  be  with  the  lowest  of  mammals, — 
the  monotremes  and  marsupials.  The  monotremes,  —  such  as 
the  duckbill  mole  and  the  spiny  ant-eater  of  Australia,  reproduce 
by  means  of  eggs  resembling  those  of  reptiles ;  the  marsupials, 
such  as  the  opossum  and  the  kangaroo,  bring  forth  their  young 
alive,  but  in  a  very  immature  condition,  and  carry  them  for 
some  time  after  birth  in  the  marsupium,  a  pouch  on  the  ventral 
side  of  the  body. 


CHAPTEK  XXI 
THE  TERTIARY 

The  Cenozoic  era.  The  last  stages  of  the  Cretaceous  are 
marked  by  a  decadence  of  the  reptiles.  By  the  end  of  that 
period  the  reptilian  forms  characteristic  of  the  time  had  become 
extinct  one  after  another,  leaving  to  represent  the  class  only 
the  types  of  reptiles  which  continue  to  modern  times.  The  day 
of  the  ammonite  and  the  belemnite  also  now  drew  to  a  close, 
and  only  a  few  of  these  cephalopods  were  left  to  survive  the 
period.  It  is  therefore  at  the  close  of  the  Cretaceous  that  the 
line  is  drawn  which  marks  the  end  of  the  Middle  Age  of  geol- 
ogy and  the  beginning  of  the  Cenozoic  era,  the  era  of  modern 
life,  —  the  Age  of  Mammals. 

In  place  of  the  giant  reptiles,  mammals  now  become  masters 
of  the  land,  appearing  first  in  generalized  types  which,  during 
the  long  ages  of  the  era,  gradually  evolve  to  higher  forms,  more 
specialized  and  ever  more  closely  resembling  the  mammals  of 
the  present.  In  the  atmosphere  the  flying  dragons  of  the  Meso- 
zoic  give  place  to  birds  and  bats.  In  the  sea,  whales,  sharks, 
and  teleost  fishes  of  modern  types  rule  in  the  stead  of  huge 
swimming  reptiles.  The  lower  vertebrates,  the  invertebrates  of 
land  and  sea,  and  the  plants  of  field  and  forest  take  on  a  modern 
aspect,  and  differ  little  more  from  those  of  to-day  than  the 
plants  and  animals  of  different  countries  now  differ  from  one 
another.  From  the  beginning  of  the  Cenozoic  era  until  now 
there  is  a  steadily  increasing  number  of  species  of  animals  and 
plants  which  have  continued  to  exist  to  the  present  time. 

The  Cenozoic  era  comprises  two  divisions,  —  the  Tertiary 
period  and  the  Quaternary  period. 

394 


THE  TERTIARY  395 

In  the  early  days  of  geology  the  formations  of  the  entire  geological 
record,  so  far  as  it  was  then  known,  were  divided  into  three  groups,  — 
the  Primary,  the  Secondary  (now  known  as  the  Mesozoic),  and  the  Ter- 
tiary. When  the  third  group  was  subdivided  into  two  systems,  the  term 
Tertiary  was  retained  for  the  first  system  of  the  two,  while  the  term 
Quaternary  was  used  to  designate  the  second. 

Divisions  of  the  Tertiary.  The  formations  of  the  Tertiary 
are  grouped  in  three  divisions,  —  the  Pliocene  (more  recent),  the 
Miocene  (less  recent),  and  the  Eocene  (the  dawn  of  the  recent). 

Each  of  these  epochs  is  long  and  complex.  Their  various  sub- 
divisions are  distinguished  each  by  its  own  peculiar  organisms, 
and  the  changes  of  physical  geography  recorded  in  their  strata. 
In  the  rapid  view  which  we  are  compelled  to  take  we  can  note 
only  a  few  of  the  most  conspicuous  events  of  the  period. 

Physical  geography  of  the  Tertiary  in  eastern  North  America. 
The  Tertiary  rocks  of  eastern  North  America  are  marine  de- 
posits and  occupy  the  coastal  lowlands  of  the  Atlantic  and  Gulf 
states  (Fig.  260).  In  New  England,  Tertiary  beds  occur  on  the 
island  of  Martha's  Vineyard,  but  not  on  the  mainland ;  hence 
the  shore  line  here  stood  somewhat  farther  out  than  now.  From 
New  Jersey  southward  the  earliest  Tertiary  sands  and  clays,  still 
unconsolidated,  leave  only  a  narrow  strip  of  the  edge  of  the 
Cretaceous  between  them  and  the  Triassic  and  crystalline  rocks 
of  the  Piedmont  oldland ;  hence  the  Atlantic  shore  here  stood 
farther  in  than  now,  and  at  the  beginning  of  the  period  the 
present  coastal  plain  was  continental  delta.  A  broad  belt  of 
Tertiary  sea-laid  limestones,  sandstones,  and  shales  surrounds 
the  Gulf  of  Mexico  and  extends  northward  up  the  Mississippi 
embayment  to  the  mouth  of  the  Ohio  Eiver ;  hence  the  Gulf 
was  then  larger  than  at  present,  and  its  waters  reached  in  a 
broad  bay  far  up  the  Mississippi  valley. 

Along  the  Atlantic  coast  the  Mesozoic  peneplain  may  be 
traced  shoreward  to  where  it  disappears  from  view  beneath 
an  unconformable  cover  of  early  Tertiary  marine  strata.  The 


396  THE  ELEMENTS  OF  GEOLOGY 

beginning  of  the  Tertiary  was  therefore  marked  by  a  subsidence. 
The  wide  erosion  surface  which  at  the  close  of  the  Mesozoic 
lay  near  sea  level  where  the  Appalachian  Mountains  and  their 
neighboring  plateaus  and  uplands  now  stand  was  lowered  gently 
along  its  seaward  edge  beneath  the  Tertiary  Atlantic  to  receive 
a  cover  of  its  sediments. 

As  the  period  progressed  slight  oscillations  occurred  from 
time  to  time.  Strips  of  coastal  plain  were  added  to  the  land, 
and  as  early  as  the  close  of  the  Miocene  the  shore  lines  of  the 
Atlantic  and  Gulf  states  had  reached  well-nigh  their  present 
place.  Louisiana  and  Florida  were  the  last  areas  to  emerge 
wholly  from  the  sea,  —  Florida  being  formed  by  a  broad  trans- 
verse upwarp  of  the  continental  delta  at  the  opening  of  the 
Miocene,  forming  first  an  island,  which  afterwards  was  joined  to 
the  mainland. 

The  Pacific  coast.  Tertiary  deposits  with  marine  fossils 
occur  along  the  western  foothills  of  the  Sierra  Nevadas,  and 
are  crumpled  among  the  mountain  masses  of  the  Coast  Ranges ; 
it  is  hence  inferred  that  the  Great  Valley  of  California  was 
then  a  border  sea,  separated  from  the  ocean  by  a  chain  of 
mountainous  islands  which  were  upridged  into  the  Coast  Ranges 
at  a  still  later  time.  Tertiary  marine  strata  are  spread  over 
the  lower  Columbia  valley  and  that  of  Puget  Sound,  showing 
that  the  Pacific  came  in  broadly  there. 

The  interior  of  the  western  United  States.  The  closing  stages 
of  the  Mesozoic  were  marked,  as  we  have  seen,  by  the  up- 
heaval of  the  Rocky  Mountains  and  other  western  ranges.  The 
bases  of  the  mountains  are  now  skirted  by  widespread  Tertiary 
deposits,  winch  form  the  highest  strata  of  the  lofty  plateaus  from 
the  level  of  whose  summits  the  mountains  rise.  Like  the  re- 
cent alluvium  of  the  Great  Valley  of  California  (p.  101),  these 
deposits  imply  low-lying  lands  when  they  were  laid,  and  there- 
fore at  that  time  the  mountains  rose  from  near  sea  level. 
But  the  height  ato  which  the  Tertiary  strata  now  stand  —  five 


397 


398  THE  ELEMENTS  OF  GEOLOGY 

thousand  feet  above  the  sea  at  Denver,  and  twice  that  height  in 
the  plateaus  of  southern  Utah  —  proves  that  the  plateaus  of 
which  the  Tertiary  strata  form  a  part  have  been  uplifted  during 
the  Cenozoic.  During  their  uplift,  warping  formed  extensive 
basins  both  east  and  west  of  the  Rockies,  and  in  these  basins 
stream-swept  and  lake-laid  waste  gathered  to  depths  of  hun- 
dreds and  thousands  of  feet,  as  it  is  accumulating  at  present  in 
the  Great  Valley  of  California  and  on  the  river  plains  of  Turkes- 
tan (p.  103).  The  Tertiary  river  deposits  of  the  High  Plains 
have  already  been  described  (p.  100).  How  widespread  are 
these  ancient  river  plains  and  beds  of  fresh-water  lakes  may  be 
seen  in  the  map  of  Figure  260. 

The  Bad  Lands.  In  several  of  the  western  states  large  areas  of  Ter- 
tiary fresh-water  deposits  have  been  dissected  to  a  maze  of  hills  whose 
steep  sides  are  cut  with  innumerable  ravines.  The  deposits  of  these 
ancient  river  plains  and  lake  beds  are  little  cemented  and  because  of 
the  dryness  of  the  climate  are  unprotected  by  vegetation  ;  hence  they 
are  easily  carved  by  the  wet- weather  rills  of  scanty  and  infrequent  rains. 
These  waterless,  rugged  surfaces  were  named  by  the  early  French 
explorers  the  Bad  Lands  because  they  were  found  so  difficult  to  traverse. 
The  strata  of  the  Bad  Lands  contain  vast  numbers  of  the  remains  of  the 
animals  of  Tertiary  times,  and  the  large  amount  of  barren  surface 
exposed  to  view  makes  search  for  fossils  easy  and  fruitful.  These 
desolate  tracts  are  therefore  frequently  visited  by  scientific  collecting 
expeditions. 

Mountain  making  in  the  Tertiary.  The  Tertiary  period 
included  epochs  when  the  earth's  crust  was  singularly  unquiet. 
From  time  to  time  on  all  the  continents  subterranean  forces 
gathered  head,  and  the  crust  was  bent  and  broken  and  upridged 
in  lofty  mountains. 

The  Sierra  Nevada  range  was  formed,  as  we  have  seen,  by 
strata  crumpling  at  the  end  of  the  Jurassic.  But  since  that 
remote  time  the  upfolded  mountains  had  been  worn  to  plains 
and  hilly  uplands,  the  remnants  of  whose  uplifted  erosion 


THE  TERTIARY  399 

surfaces  may  now  be  traced  along  the  western  mountain  slopes. 
Beginning  late  in  the  Tertiary,  the  region  was  again  affected  by 
mountain-making  movements.  A  series  of  displacements  along 
a  profound  fault  on  the  eastern  side  tilted  the  enormous  earth 
block  of  the  Sierras  to  the  west,  lifting  its  eastern  edge  to  form 
the  lofty  crest  and  giving  to  the  range  a  steep  eastern  front  and 
a  gentle  descent  toward  the  Pacific. 

The  Coast  Ranges  also  have  had  a  complex  history  with  many  vicis- 
situdes. The  earliest  foldings  of  their  strata  belong  to  the  close  of  the 
Jurassic,  but  it  was  not  until  the  end  of  the  Miocene  that  the  line  of 
mountainous  islands  and  the  heavy  sediments  which  had  been  deposited 
on  their  submerged  flanks  were  crushed  into  a  continuous  mountain 
chain.  Thick  Pliocene  beds  upon  their  sides  prove  that  they  were 
depressed  to  near  sea  level  during  the  later  Tertiary.  At  the  close  of 
the  Pliocene  the  Coast  Ranges  rose  along  with  the  upheaval  of  the 
Sierra,  and  their  gradual  uplift  has  continued  to  the  present  time. 

The  numerous  north-south  ranges  of  the  Great  Basin  and  the  Mount 
Saint  Elias  range  of  Alaska  were  also  uptilted  during  the  Tertiary. 

During  the  Tertiary  period  many  of  the  loftiest  mountains 
of  the  earth  —  the  Alps,  the  Apennines,  the  Pyrenees,  the 
Atlas,  the  Caucasus,  and  the  Himalayas  —  received  the  uplift 
to  which  they  owe  most  of  their  colossal  bulk  and  height,  as 
portions  of  the  Tertiary  sea  beds  now  found  high  upon  their 
flanks  attest.  In  the  Himalayas,  Tertiary  marine  limestones 
occur  sixteen  thousand  five  hundred  feet  above  sea  level. 

Volcanic  activity  in  the  Tertiary.  The  vast  deformations  of 
the  Tertiary  were  accompanied  on  a  corresponding  scale  by  out- 
pourings of  lava,  the  outburst  of  volcanoes,  and  the  intrusion  of 
molten  masses  within  the  crust.  In  the  Sierra  Nevadas  the 
Miocene  river  gravels  of  the  valleys  of  the  western  slope,  with 
their  placer  deposits  of  gold,  were  buried  beneath  streams  of 
lava  and  beds  of  tuff  (Fig.  258).  Volcanoes  broke  forth  along 
the  Eocky  Mountains  and  on  the  plateaus  of  Utah,  New  Mexico, 
and  Arizona. 


400 


THE  ELEMENTS  OF  GEOLOGY 


Mount  Shasta  and  the  immense  volcanic  piles  of  the  Cascades 
date  from  this  period.  The  mountain  basin  of  the  Yellowstone 
Park  was  filled  to  a  depth  of  several  thousand  feet  with  tuffs 
and  lavas,  the  oldest  dating  as  far  back  as  the  beginning  of  the 
Tertiary.  Crandall  volcano  (p.  263)  was  reared  in  the  Miocene 
and  the  latest  eruptions  of  the  Park  are  far  more  recent. 

The  Columbia  and  Snake  River  lavas.  Still  more  impor- 
tant is  the  plateau  of  lava,  more  than  two  hundred  thousand 
square  miles  in  area,  extending  from  the  Yellowstone  Park  to  the 

Cascade    Mountains, 

which  has  been  built 
from  Miocene  times  to 
the  present. 

Over  this  plateau, 
which  occupies  large  por- 
tions of  Idaho,  Washing- 
ton, and  Oregon,  and 
extends  into  northern 
California  and  Nevada, 
the  country  rock  is  ba- 
saltic lava.  For  thousands  of  square  miles  the  surface  is  a  lava  plain 
which  meets  the  boundary  mountains  as  a  lake  or  sea  meets  a  rugged 
and  deeply  indented  coast.  The  floods  of  molten  rock  spread  up  the 
mountain  valleys  for  a  score  of  miles  and  more,  the  intervening  spurs 
rising  above  the  lava  like  long  peninsulas,  while  here  and  there  an 
isolated  peak  was  left  to  tower  above  the  inundation  like  an  island 
off  a  submerged  shore. 

The  rivers  which  drain  the  plateau — the  Snake,  the  Columbia,  and 
their  tributaries  —  have  deeply  trenched  it,  yet  their  canyons,  which  reach 
the  depth  of  several  thousand  feet,  have  not  been  worn  to  the  base  of  the 
lava  except  near  the  margin  and  where  they  cut  the  summits  of  mountains 
drowned  beneath  the  flood.  Here  and  there  the  plateau  has  been 
deformed.  It  lias  been  upbent  into  great  folds,  and  broken  into  immense 
blocks  of  bedded  lava,  forming  mountain  ranges,  which  run  parallel 
with  the  Pacific  coast  line.  On  the  edges  of  these  tilted  blocks  the 
thickness  of  the  lava  is  seen  to  be  fully  five  thousand  feet.  The  plateau 


FIG.  341.    Lava  Plateau  with  Lava  Domes 
in  the  Distance,  Idaho 


THE  TERTIARY  401 

has  been  built,  like  that  of  Iceland  (p.  242),  of  innumerable  overlapping 
sheets  of  lava.  On  the  canyon  walls  they  weather  back  in  horizontal 
terraces  and  long  talus  slopes.  One  may  distinguish  each  successive 
flow  by  its  dense  central  portion,  often  jointed  with  large  vertical  col- 
umns, and  the  upper  portion  with  its  mass  of  confused  irregular  columns 
and  scoriaceous  surface.  The  average  thickness  of  the  flows  seems  to 
be  about  seventy-five  feet. 

The  plateau  was  long  in  building.  Between  the  layers  are  found  in 
places  old  soil  beds  and  forest  grounds  and  the  sediments  of  lakes. 
Hence  the  interval  between  the  flows  in  any  locality  was  sometimes 
long  enough  for  clays  to  gather  in  the  lakes  which  filled  depressions  in 
the  surface.  Again  and  again  the  surface  of  the  black  basalt  was 
reddened  by  oxidation  and  decayed  to  soil,  and  forests  had  time  to  grow 
upon  it  before  the  succeeding  inundation  sealed  the  sediments  and  soils 
away  beneath  a  sheet  of  stone.  Near  the  edges  of  the  lava  plain,  rivers 
from  the  surrounding  mountains  spread  sheets  of  sand  and  gravel  on 
the  surface  of  one  flow  after  another.  These  pervious  sands,  interbedded 
with  the  lava,  become  the  aquifers  of  artesian  wells. 

In  places  the  lavas  rest  on  extensive  lake  deposits,  one  thousand  feet 
deep,  and  Miocene  in  age  as  their  fossils  prove.  It  is  to  the  middle  Ter- 
tiary, then,  that  the  earliest  flows  and  the  largest  bulk  of  the  great  inun- 
dation belong.  So  ancient  are  the  latest  floods  in  the  Columbia  basin 
that  they  have  weathered  to  a  residual  yellow  clay  from  thirty  to  sixty 
i  feet  in  depth  and  marvelously  rich  in  the  mineral  substances  on  which 
plants  feed. 

In  the  Snake  River  valley  the  latest  lavas  are  much  younger.  Their 
surfaces  are  so  fresh  and  imdecayed  that  here  the  effusive  eruptions  may 
well  have  continued  to  within  the  period  of  human  history.  Low  lava 
domes  like  those  of  Iceland  mark  where  last  the  basalt  outwelled  and 
spread  far  and  wide  before  it  chilled  (Fig.  341).  In  places  small  mounds 
of  scoria  show  that  the  eruptions  were  accompanied  to  a  slight  degree 
by  explosions  of  steam.  So  fluid  was  this  superheated  lava  that  recent 
flows  have  been  traced  for  more  than  fifty  miles. 

The  rocks  underlying  the  Columbia  lavas,  where  exposed  to  view, 
[  are  seen  to  be  cut  by  numerous  great  dikes  of  dense  basalt,  which  mark 
the  fissures  through  which  the  molten  rock  rose  to  the  surface. 

The  Tertiary  included  times  of  widespread  and  intense  vol- 
canic action  in  other  continents  as  well  as  in  North  America, 


402 


THE  ELEMENTS  OF  GEOLOGY 


In  Europe,  Vesuvius  (Fig.  231)  and  Etna  began  their  career  as 
submarine  volcanoes  in  connection  with  earth  movements  which 
finally  lifted  Pliocene  deposits  in  Sicily  to  their  present  height,  — 
four  thousand  feet  above  the  sea.  Volcanoes  broke  forth  in  central 
France  and  southern  Germany,  in  Hungary  and  the  Carpathians. 
Innumerable  fissures  opened  in  the  crust  from  the  north  of  Ire- 
land and  the  western  islands  of  Scotland  to  the  Faroes,  Iceland, 
and  even  to  arctic  Greenland  ;  and  here  great  plateaus  were  built 

of  flows  of  basalt  similar 
to  that  of  the  Columbia 
River.  In  India,  at  the 
opening  of  the  Tertiary, 
there  had  been  an  out- 
welling  of  basalt,  flood- 
ing to  a  depth  of  thou- 
sands of  feet  two  hundred 
thousand  square  miles  of 
the  northwestern  part  of 
the  peninsula  (Fig.  342), 
and  similar  inundations 
of  lava  occurred  where 
are  now  the  table-lands 
of  Abyssinia.  From  the 
middle  Tertiary  on,  Asia 
Minor,  Arabia,  and  Persia 
were  the  scenes  of  volcanic  action.  In  Palestine  the  rise  of  the 
uplands  of  Judea  at  the  close  of  the  Eocene,  and  the  down- 
faulting  of  the  Jordan  valley  (p.  221)  were  followed  by  vol- 
canic outbursts.  In  comparison  with  the  middle  Tertiary,  the 
present  is  a  time  of  volcanic  inactivity  and  repose. 

Erosion  of  Tertiary  mountains  and  plateaus.  The  mountains 
and  plateaus  built  at  various  times  during  the  Tertiary  and  at 
its  commencement  have  been  profoundly  carved  by  erosive 
agents.  The  Sierra  Nevada  Mountains  have  been  dissected  on 


FIG.  342.    Map  showing  the  Lava  Sheet 
(shaded  area)  of  Western  India 


THE  TERTIARY  403 

the  western  slope  by  such  canyons  as  those  of  King's  River  and 
the  Yosemite.  Six  miles  of  strata  have  been  denuded  from  parts 
of  the  Wasatch  Mountains  since  their  rise  at  the  beginning  of 
the  era.  From  the  Colorado  plateaus,  whose  uplift  dates  from  the 
same  time,  there  have  been  stripped  off  ten  thousand  feet  of 
strata  over  thousands  of  square  miles,  and  the  colossal  canyon 
of  the  Colorado  has  been  cut  after  this  great  denudation  had 
been  mostly  accomplished  (Fig.  130). 

On  the  eastern  side  of  the  continent,  as  we  have  seen,  a 
broad  peneplain  had  been  developed  by  the  close  of  the  Creta- 
ceous.* The  remnants  of  this  old  erosion  surface  are  now  found 
upwarped  to  various  heights  in  different  portions  of  its  area. 
In  southern  New  England  it  now  stands  fifteen  hundred  feet 
•  above  the  sea  in  western  Massachusetts,  declining  thence  south- 
ward and  eastward  to  sea  level  at  the  coast.  In  southwestern 
Virginia  it  has  been  lifted  to  four  thousand  feet  above  the  sea. 
Manifestly  this  upwarp  occurred  since  the  peneplain  was  formed ; 
it  is  later  than  the  Mesozoic,  and  the  vast  dissection  which  the 
peneplain  has  suffered  since  its  uplift  must  belong  to  the  suc- 
cessive cycles  of  Cenozoic  time. 

Revived  by  the  uplift,  the  streams  of  the  area  trenched  it  as 
deeply  as  its  elevation  permitted,  and  reaching  grade,  opened  up 
wide  valleys  and  new  peneplains  in  the  softer  rocks.  The  Con- 
necticut valley  is  Tertiary  in  age,  and  in  the  weak  Triassic 
sandstones  (p.  370)  has  been  widened  in  places  to  fifteen  miles. 
Dating  from  the  same  time  are  the  valleys  of  the  Hudson,  the 
Susquehanna,  the  Delaware,  the  Potomac,  and  the  Shenandoah. 

In  Pennsylvania  and  the  states  lying  to  the  south  the  Meso- 
zoic peneplain  lies  along  the  summits  of  the  mountain  ridges. 
On  the  surface  of  this  ancient  plain,  Tertiary  erosion  etched  out 
the  beautifully  regular  pattern  of  the  Allegheny  mountain 
ridges  and  their  intervening  valleys.  The  weaker  strata  of  the 
long,  regular  folds  were  eroded  into  longitudinal  valleys,  while 
the  hard  Paleozoic  sandstones,  such  as  the  Medina  (p.  335) 


404 


THE  ELEMENTS  OF  GEOLOGY 


and  the  Pocono  (p.  350),  were  left  in  relief  as  bold  mountain 
walls  whose  even  crests  rise  to  the  common  level  of  the  ancient 
plain.  From  Virginia  far  into  Alabama  the  great  Appalachian 
valley  was  opened  to  a  width  in  places  of  fifty  miles  and  more, 
along  a  belt  of  intensely  folded  and  faulted  strata  where  once 
was  the  heart  of  the  Appalachian  Mountains.  In  Figure  70 


FIG.  343.    Diagram  of  the  Allegheny  Mountains,  Pennsylvania 
From  Davis'  Elementary  Physical  Geography 

the  summit  of  the  Cumberland  plateau  (db)  marks  the  level 
of  the  Mesozoic  peneplain,  while  the  lower  erosion  levels  are 
Tertiary  and  Quaternary  in  age. 

LIFE  OF  THE  TERTIARY  PERIOD 

Vegetation  and  climate.  The  highest  plants  in  structure,  the 
dicotyls  (such  as  our  deciduous  forest  trees)  and  the  monocotyls 
(represented  by  the  palms),  were  introduced  during  the  Creta- 
ceous. The  vegetable  kingdom  reached  its  culmination  before 
the  animal  kingdom,  and  if  the  dividing  line  between  the  Meso- 
zoic and  the  Cenozoic  were  drawn  according  to  the  progress  of 


THE  TERTIARY  405 

plant  life,  the  Cretaceous  instead  of  the  Tertiary  would  be  made 
the  opening  period  of  the  modern  era. 

The  plants  of  the  Tertiary  belonged,  for  the  most  part,  to  gen- 
era now  living;  but  their  distribution  was  very  different  from 
that  of  the  flora  of  to-day.  In  the  earlier  Tertiary,  palms  flour- 
ished over  northern  Europe,  and  in  the  northwestern  United 
States  grew  the  magnolia  and  laurel,  along  with  the  walnut, 
oak,  and  elm.  Even  in  northern  Greenland  and  in  Spitz- 
bergen  there  were  lakes  covered  with  water  lilies  and  sur- 
rounded by  forests  of  maples,  poplars,  limes,  the  cypress  of  our 
southern  states,  and  noble  sequoias  similar  to  the  "  big  trees  " 
and  redwoods  of  California.  A  warm  climate  like  that  of  the 
Mesozoic,  therefore,  prevailed  over  North  America  and  Europe, 
extending  far  toward  the  pole.  In  the  later  Tertiary  the  climate 
gradually  became  cooler.  Palms  disappeared  from  Europe,  and 
everywhere  the  aspect  of  forests  and  open  lands  became  more 
like  that  of  to-day.  Grasses  became  abundant,  furnishing  a  new 
£ood  for  herbivorous  animals. 

Animal  life  of  the  Tertiary.  Little  needs  to  be  said  of  the 
Tertiary  invertebrates,  so  nearly  were  they  like  the  invertebrates 
of  the  present.  Even  in  the  Eocene,  about  five  per  cent  of 
marine  shells  were  of  species  still  living,  and  in  the  Pliocene 
the  proportion  had  risen  to  more  than  one  half. 

Fishes  were  of  modern  types.  Teleosts  were  now  abundant. 
The  ocean  teemed  with  sharks,  some  of  them  being  voracious 
monsters  seventy-five  feet  and  even  more  in  length,  with  a  gape 
of  jaw  of  six  feet,  as  estimated  by  the  size  of  their  enormous 
sharp-edged  teeth. 

Snakes  are  found  for  the  first  time  in  the  early  Tertiary. 
These  limbless  reptiles,  evolved  by  degeneration  from  lizardlike 
ancestors,  appeared  in  nonpoisonous  types  scarcely  to  be  dis- 
tinguished from  those  of  the  present  day. 

Mammals  of  the  early  Tertiary.  The  fossils  of  continental 
deposits  of  the  earliest  Eocene  show  that  a  marked  advance  had 


406         THE  ELEMENTS  OF  GEOLOGY 

now  been  made  in  the  evolution  of  the  Mammalia.    The  higher 
mammals  had  appeared,  and  henceforth  the  lower  mammals  — 
the  monotremes  and  the  marsupials  —  are  reduced  to  a  subordi- 
nate place. 

These  first  true  mammals  were  archaic  and  generalized  in 
structure.  Their  feet  were  of  the  primitive  type,  with  five  toes 
of  about  equal  length.  They  were  also  plantigrades,  —  that  is, 
they  touched  the  ground  with  the  sole  of  the  entire  foot  from 
toe  to  heel.  No  foot  had  yet  become  adapted  to  swift  running 
by  a  decrease  in  the  number  of  digits  and  by  lifting  the  heel  and 


FIG.  344.    Phenacodus 

sole  so  that  only  the  toes  touch  the  ground,  —  a  tread  called 
digitigrade.  Nor  was  there  yet  any  foot  like  that  of  the  cats, 
with  sharp  retractile  claws  adapted  to  seizing  and  tearing  the 
prey.  The  forearm  and  the  lower  leg  each  had  still  two  separate 
bones  (ulna  and  radius,  fibula  and  tibia),  neither  pair  having 
been  replaced  with  a  single  strong  bone,  as  in  the  leg  of  the  horse. 
The  teeth  also  were  primitive  in  type  and  of  full  number.  The 
complex  heavy  grinders  of  the  horse  and  elephant,  the  sharp  cut- 
ting teeth  of  the  carnivores,  and  the  chopping  teeth  of  the  grass 
eaters  were  all  still  to  come. 

Phenacodus   is   a  characteristic   genus   of  the  early  Eocene,   whose 
species  varied  in  size  from  that  of  a  bulldog  to  that  of  an  animal  a  little 


THE  TERTIARY  407 

larger  than  a  sheep.  Its  feet  were  primitive,  and  their  five  toes  bore  nails 
intermediate  in  form  between  a  claw  and  a  hoof.  The  archaic  type  of 
teeth  indicates  that  the  animal  was  omnivorous  in  diet.  A  cast  of  the 
brain  cavity  shows  that,  like  its  associates  of  the  time,  its  brain  was 
extremely  small  and  nearly  smooth,  having  little  more  than  traces  of 
convolutions. 

The  long  ages  of  the  Eocene  and  the  following  epochs  of  the 
Tertiary  were  times  of  comparatively  rapid  evolution  among  the 
Mammalia.  The  earliest  forms  evolved  along  diverging  lines 
toward  the  various  specialized  types  of  hoofed  mammals,  rodents, 
carnivores,  proboscidians,  the  primates,  and  the  other  mammalian 
orders  as  we  know  them  now.  We  must  describe  the  Tertiary 
mammals  very  briefly,  tracing  the  lines  of  descent  of  only  a  few 
of  the  more  familiar  mammals  of  the  present. 

The  horse.  The  pedigree  of  the  horse  runs  back  into  the 
early  Eocene  through  many  genera  and  species  to  a  five-toed,1 
short-legged  ancestor  little  bigger  than  a  cat.  Its  descendants 
gradually  increased  in  stature  and  became  better  and  better 
adapted  to  swift  running  to  escape  their  foes.  The  leg  became 
longer,  and  only  the  tip  of  the  toes  struck  the  ground,  The 
middle  toe  (digit  number  three),  originally  the  longest  of  the 
five,  steadily  enlarged,  while  the  remaining  digits  dwindled  and 
disappeared.  The  inner  digit,  corresponding  to  the  great  toe  and 
thumb,  was  the  first  to  go.  Next  number  five,  the  little  finger, 
was  also  dropped.  By  the  end  of  the  Eocene  a  three-toed  genus 
of  the  horse  family  had  appeared,  as  large  as  a  sheep.  The  hoof 
of  digit  number  three  now  supported  most  of  the  weight,  but 
the  slender  hoofs  of  digits  two  and  four  were  still  serviceable. 
In  the  Miocene  the  stature  of  the  ancestors  of  the  horse  increased 
to  that  of  a  pony.  The  feet  were  still  three-toed,  but  the  side 
hoofs  were  now  mere  dewclaws  and  scarcely  touched  the  ground. 
The  evolution  of  the  family  was  completed  in  the  Pliocene. 

1  Or,  more  accurately,  with  four  perfect  toes  and  a  rudimentary  fifth  corre- 
sponding to  the  thumb. 


408 


THE  ELEMENTS  OF  GEOLOGY 


Equus 
Pleistocene 

and 
Recent 


Pliohippus 
Pliocene 


Protohippus 

Lower 
Pliocene 


Miohippus 
Miocene 


Mesohippus 
Lower 
Miocene 


Orohippus 
Eocene 


ABC 
FIG.  345.    Development  of  Forefoot 
(A),  the  Forearm  (E),  and  Molar 
(C),  of  the  Horse  Family 


The  middle  toe  was  enlarged  still 
more,  the  side  toes  were  dropped, 
and  the  palm  and  foot  bones 
which  supported  them  were  re- 
duced to  splints. 

While  these  changes  were  in 
progress  the  radius  and  ulna  of 
the  fore  limb  became  consoli- 
dated to  a  single  bone ;  and  in 
the  hind  limb  the  fibula  dwindled 
to  a  splint,  while  the  tibia  was 
correspondingly  enlarged.  The 
molars  also  gradually  lengthened, 
and  became  more  and  more  com- 
plex on  their  grinding  surface ; 
the  neck  became  longer ;  the 
brain  steadily  increased  in  size 
and  its  convolutions  became 
more  abundant.  The  evolution 
of  the  horse  has  made  for  greater 
fleetness  and  intelligence. 

The  rhinoceros  and  tapir. 
These  animals,  which  are  grouped 
with  the  horse  among  the  odd- 
toed  (perissodactyl)  mammals,  are 
now  verging  toward  extinction. 
In  the  rhinoceros,  evolution 
seems  to  have  taken  the  opposite 
course  from  that  of  the  horse. 
As  the  animal  increased  in  size 
it  became  more  clumsy,  its  limbs 
became  shorter  and  more  mas- 
sive, and,  perhaps  because  of  its 
great  weight,  the  number  of  digits 


THE  TERTIARY  409 

were  not  reduced  below  the  number  three.  Like  other  large 
herbivores,  the  rhinoceros,  too  slow  to  escape  its  enemies  by 
flight,  learned  to  withstand  them.  It  developed  as  its  means 
of  defense  a  nasal  horn. 

Peculiar  offshoots  of  the  line  appeared  at  various  times  in  the  Ter- 
tiary. A  rhinoceros,  semiaquatic  in  habits,  with  curved  tusks,  resembling 
in  aspect  the  hippopotamus,  lived  along  the  water  courses  of  the  plains 
east  of  the  Rockies,  and  its  bones  are  now  found  by  the  thousands  in 
the  Miocene  of  Kansas.  Another  developed  along  a  line  parallel  to 
that  of  the  horse,  and  herds  of  these  light-limbed  and  swift-footed  run- 
ning rhinoceroses  ranged  the  Great  Plains  from  the  Dakotas  southward. 


FIG.  346.  A  Tertiary  Mastodon 

The  tapirs  are  an  ancient  family  which  has  changed  but 
little  since  it  separated  from  the  other  perissodactyl  stocks  in 
the  early  Tertiary.  At  present,  tapirs  are  found  only  in  South 
America  and  southern  Asia,  —  a  remarkable  distribution  which 
we  could  not  explain  were  it  not  that  the  geological  record 
shows  that  during  Tertiary  times  tapirs  ranged  throughout  the 
northern  hemisphere,  making  their  way  to  South  America  late 
in  that  period.  During  the  Pleistocene  they  became  extinct  over 
all  the  intervening  lands  between  the  widely  separated  regions 
where  now  they  live.  The  geographic  distribution  of  animals, 
as  well  as  their  relationships  and  origins,  can  be  understood 
only  through  a  study  of  their  geological  history. 


410 


THE  ELEMENTS  OF  GEOLOGY 


The  proboscidians.  This  unique  order  of  hoofed  mammals,  of 
which  the  elephant  is  the  sole  survivor,  began,  so  far  as  known, 
in  the  Eocene,  in  Egypt,  with  a  piglike  ancestor  the  size  of  a 
small  horse,  with  cheek  teeth  like  the  Mastodon's,  but  want- 
ing both  trunk  and  tusks.  A  proboscidian  came  next  with 

four  short  tusks,  and  in  the  Miocene 
there  followed  a  Mastodon  (Eig.  346) 
armed  with  two  pairs  of  long,  straight 
tusks  on  which  rested  a  flexible  pro- 
boscis. 

The  Dinothere  was  a  curious  offshoot 
of  the  line,  which  developed  in  the  Mio- 
cene in  Europe.  In  this  immense  pro- 
boscidian, whose  skull  was  three  feet 
long,  the  upper  pair  of  tusks  had  disap- 
peared, and  those  of  the  lower  jaw  were 
bent  down  with  a  backward  curve  in 
FIG.  347.  Head  of  Dinothere  walrus  fashion. 

In  the  true  elephants,  which  do  not  appear  until  near  the  close 
of  the  Tertiary,  the  lower  jaw  loses  its  tusks  and  the  grinding 
teeth  become  exceedingly  complex  in  structure.  The  grinding 

teeth  of  the  mastodon 
had  long  roots  and  low 
crowns  crossed  by  four 
or  five  peaked  enameled 
ridges.  In  the  teeth  of 
the  true  elephants  the 
crown  has  become  deep, 
and  the  ridges  of  enamel 
have  changed  to  numerous  upright,  platelike  folds,  their  inter- 
spaces filled  with  cement.  The  two  genera  —  Mastodon  and 
Elephant  —  are  connected  by  species  whose  teeth  are  interme- 
diate in  pattern.  The  proboscidians  culminated  in  the  Pliocene, 
when  some  of  the  giant  elephants  reached  a  height  of  fourteen  feet. 


FIG.  348.    Crown  of  Mastodon  Tooth 


THE  TERTIARY 


411 


The  artiodactyls  comprise  the  hoofed  Mammalia  which  have 
an  even  number  of  toes,  such  as  cattle,  sheep,  and  swine.  Like 
the  perissodactyls, 
they  are  descended 
from  the  primitive 
five-toed  planti- 
grade mammals  of 
the  lowest  Eocene. 
In  their  evolution, 
digit  number  one 
was  first  dropped, 
and  the  middle  pair 
became  larger  and  more  massive,  while  the  side  digits,  numbers 
two  and  five,  became  shorter,  weaker,  and  less  serviceable.  The 


FIG.  349.   Tooth  of  an  Extinct  Elephant, 
the  Mammoth 


m\J      t/JV 
A  BCD 

FIG.  350.   Evolution  of  the  Artiodactyl  Foot,  illustrated  by 

Existing  Families 
A,  pig;  B,  roebuck;  C,  sheep;  D,  camel 

four-toed  artiodactyls  culminated  in  the  Tertiary ;  at  present 
they  are  represented  only  by  the  hippopotamus  and  the  hog. 


412         THE  ELEMENTS  OF  GEOLOGY 

Along  the  main  line  of  the  evolution  of  the  artiodactyls  the 
side  toes,  digits  two  and  five,  disappeared,  leaving  as  proof  that 
they  once  existed  the  corresponding  bones  of  palm  and  sole  as 
splints.  The  two-toed  artiodactyls,  such  as  the  camels,  deer, 
cattle,  and  sheep,  are  now  the  leading  types  of  the  herbivores. 
Swine  and  peccaries  are  two  branches  of  a  common  stock, 
the  first  developing  in  the  Old  World  and  the  second  in  the 
New.  In  the  Miocene  a  noticeable  offshoot  of  the  line  was  a 
gigantic  piglike  brute,  a  root  eater,  with  a  skull  a  yard  in  length, 
whose  remains  are  now  found  in  Colorado  and  South  Dakota. 

Camels  and  llamas.  The  line  of  camels  and  llamas  developed  in 
North  America,  where  the  successive  changes  from  an  early  Eocene 
ancestor,  no  larger  than  a  rabbit,  are  traced  step  by  step  to  the  present 
forms,  as  clearly  as  is  the  evolution  of  the  horse.  In  the  late  Miocene 
some  of  the  ancestral  forms  migrated  to  the  Old  World  by  way  of  a  land 
connection  where  Bering  Strait  now  is,  and  there  gave  rise  to  the  camels 
and  dromedaries.  Others  migrated  into  South  America,  which  had  now 
been  connected  with  our  own  continent,  and  these  developed  into  the 
llamas  and  guanacos,  while  those  of  the  race  which  remained  in  North 
America  became  extinct  during  the  Pleistocene. 

Some  peculiar  branches  of  the  camel  stem  appeared  in  North  America. 
In  the  Pliocene  arose  a  llama  with  the  long  neck  and  limbs  of  a  giraffe, 
whose  food  was  cropped  from  the  leaves  and  branches  of  trees.  Far 
more  generalized  in  structure  was  the  Oreodon,  an  animal  related  to 
the  camels,  but  with  distinct  affinities  also  with  other  lines,  such  as 
those  of  the  hog  and  deer.  These  curious  creatures  were  much  like  the 
peccary  in  appearance,  except  for  their  long  tails.  In  the  middle  Eocene 
they  roamed  in  vast  herds  from  Oregon  to  Kansas  and  Nebraska. 

The  ruminants.  This  division  of  the  artiodactyls  includes 
antelopes,  deer,  oxen,  bison,  sheep,  and  goats,  —  all  of  which  be- 
long to  a  common  stock  which  took  its  rise  in  Europe  in  the  upper 
Eocene  from  ancestral  forms  akin  to  those  of  the  camels.  In 
the  Miocene  the  evolution  of  the  two-toed  artiodactyl  foot  was 
well-nigh  completed.  Bonelike  growths  appeared  on  the  head, 
and  the  two  groups  of  the  ruminants  became  specialized,  —  the 


THE  TERTIARY  413 

deer  with  bony  antlers,  shed  and  renewed  each  year,  and  the 
ruminants  with  hollow  horns,  whose  two  bony  knobs  upon 
the  skull  are  covered  with  permanent,  pointed,  horny  sheaths. 

The  ruminants  evolved  in  the  Old  World,  and  it  was  not  until  the 
later  Miocene  that  the  ancestors  of  the  antelope  and  of  some  deer  found 
their  way  to  North  America.  Mountain  sheep  and  goats,  the  bison  and 
most  of  the  deer,  did  not  arrive  until  after  the  close  of  the  Tertiary, 
and  sheep  and  oxen  were  introduced  by  man. 

The  hoofed  mammals  of  the  Tertiary  included  many  offshoots  from 
the  main  lines  which  we  have  traced.  Among  them  were  a  number  of 
genera  of  clumsy,  ponderous  brutes,  some  almost  elephantine  in  their 
bulk. 

The  carnivores.  The  ancestral  lines  of  the  families  of  the 
flesh  eaters  —  such  as  the  cats  (lions,  tigers,  etc.),  the  bears,  the 
hyenas,  and  the  dogs  (including  wolves  and  foxes)  —  converge 
in  the  creodonts  of  the  early  Eocene,  —  an  order  so  generalized 
that  it  had  affinities  not  only  with  the  carnivores  but  also  with 
the  insect  eaters,  the  marsupials,  and  the  hoofed  mammals  as 
well.  From  these  primitive  flesh  eaters,  with  small  and  simple 
brains,  numerous  small  teeth,  and  plantigrade  tread,  the  different 
families  of  the  carnivores  of  the  present  have  slowly  evolved. 

Dogs  and  bears.  The  dog  family  diverged  from  the  creodonts 
late  in  the  Eocene,  and  divided  into  two  branches,  one  of  which 
evolved  the  wolves  and  the  other  the  foxes.  An  offshoot  gave 
rise  to  the  family  of  the  bears,  and  so  closely  do  these  two 
families,  now  wide  apart,  approach  as  we  trace  them  back  in 
Tertiary  times  that  the  Amphicyon,  a  genus  doglike  in  its  teeth 
and  bearlike  in  other  structures,  is  referred  by  some  to  the  dog 
and  by  others  to  the  bear  family.  The  well-known  plantigrade 
tread  of  bears  is  a  primitive  characteristic  which  has  survived 
from  their  creodont  ancestry. 

Cats.  The  family  of  the  cats,  the  most  highly  specialized  of  all 
the  carnivores,  divided  in  the  Tertiary  into  two  main  branches. 
One,  the  saber-tooth  tigers  (Fig.  351),  which  takes  its  name  from 


414          THE  ELEMENTS  OF  GEOLOGY 

their  long,  saberlike,  sharp-edged  upper  canine  teeth,  evolved  a 
succession  of  genera  and  species,  among  them  some  of  the  most 
destructive  beasts  of  prey  which  ever  scourged  the  earth.  They 
were  masters  of  the  entire  northern  hemisphere  during  the 
middle  Tertiary,  but  in  Europe  during  the  Pliocene  they  declined, 
from  unknown  causes,  and  gave  place  to  the  other  branch  of 
cats,  —  which  includes  the  lions,  tigers,  and  leopards.  In  the 
Americas  the  saber-tooth  tigers  long  survived  the  epoch. 


FIG.  351.    Saber-Tooth  Tiger 

Marine  mammals.  The  carnivorous  mammals  of  the  sea  - 
whales,  seals,  walruses,  etc.  —  seem  to  have  been  derived  from 
some  of  the  creodonts  of  the  early  Tertiary  by  adaptation  to 
aquatic  life.  Whales  evolved  from  some  land  ancestry  at  a  very 
early  date  in  the  Tertiary ;  in  the  marine  deposits  of  the  Eocene 
are  found  the  bones  of  the  Zeuglodon,  a  whalelike  creature 
seventy  feet  in  length. 

Primates.  This  order,  which  includes  lemurs,  monkeys,  apes, 
and  man,  seems  to  have  sprung  from  a  creodont  or  insectivorous 
ancestry  in  the  lower  Eocene.  Lemur-like  types,  with  small, 
smooth  brains,  were  abundant  in  the  United  States  in  the 
early  Tertiary,  but  no  primates  have  been  found  here  in  the  mid- 
dle Tertiary  and  later  strata.  In  Europe  true  monkeys  were 


THE  TERTIARY  415 

introduced  in  the  Miocene,  and  were  abundant  until  the  close 
of  the  Tertiary,  when  they  were  driven  from  the  continent  by 
the  increasing  cold. 

Advance  of  the  Mammalia  during  the  Tertiary.  During  the 
several  millions  of  years  comprised  in  Tertiary  tune  the  mam- 
mals evolved  from  the  lowly,  simple  types  which  tenanted  the 
earth  at  the  beginning  of  the  period,  into  the  many  kinds  of 
highly  specialized  mammals  of  the  Pleistocene  and  the  present, 
each  with  the  various  structures  of  the  body  adapted  to  its  own 
peculiar  mode  of  life.  The  swift  feet  of  the  horse,  the  horns  of 
cattle  and  the  antlers  of  the  deer,  the  lion's  claws  and  teeth, 
the  long  incisors  of  the  beaver,  the  proboscis  of  the  elephant, 
were  all  developed  in  Tertiary  times.  In  especial  the  brain  of 
the  Tertiary  mammals  constantly  grew  larger  relatively  to  the 
size  of  body,  and  the  higher  portion  of  the  brain  —  the  cerebral 
lobes  —  increased  in  size  in  comparison  with  the  cerebellum. 
Some  of  the  hoofed  mammals  now  have  a  brain  eight  or  ten 
times  the  size  of  that  of  their  early  Tertiary  predecessors  of  equal 
bulk.  Nor  can  we  doubt  that  along  with  the  increasing  size  of 
brain  went  a  corresponding  increase  in  the  keenness  of  the 
senses,  in  activity  and  vigor,  and  in  intelligence. 


CHAPTEK  XXII 
THE   QUATERNARY 

The  last  period  of  geological  history,  the  Quaternary,  may  be 
said  to  have  begun  when  all,  or  nearly  all,  living  species  of 
mollusks  and  most  of  the  existing  mammals  had  appeared. 
It  is  divided  into  two  great  epochs.  The  first,  the  Pleistocene  or 
Glacial  epoch,  is  marked  off  from  the  Tertiary  by  the  occupation 
of  the  northern  parts  of  North  America  and  Europe  by  vast  ice 
sheets ;  the  second,  the  Recent  epoch,  began  with  the  disappear- 
ance of  the  ice  sheets  from  these  continents,  and  merges  into 
the  present  time. 

THE  PLEISTOCENE  EPOCH 

We  now  come  to  an  episode  of  unusual  interest,  so  different 
was  it  from  most  of  the  preceding  epochs  and  from  the  present, 
and  so  largely  has  it  influenced  the  conditions  of  man's  life. 

The  records  of  the  Glacial  epoch  are  so  plain  and  full  that 
we  are  compelled  to  believe  what  otherwise  would  seem  almost 
incredible,  —  that  following  the  mild  climate  of  the  Tertiary 
came  a  succession  of  ages  when  ice  fields,  like  that  of  Green- 
land, shrouded  the  northern  parts  of  North  America  and  Europe 
and  extended  far  into  temperate  latitudes. 

The  drift.  Our  studies  of  glaciers  have  prepared  us  to  decipher 
and  interpret  the  history  of  the  Glacial  epoch,  as  it  is  recorded 
in  the  surface  deposits  known  as  the  drift.  Over  most  of  Canada 
and  the  northern  states  this  familiar  formation  is  exposed  to 
view  in  nearly  all  cuttings  which  pass  below  the  surface  soil. 
The  drift  includes  two  distinct  classes  of  deposits, — the  unstrati- 
fied  drift  laid  down  by  glacier  ice,  and  the  stratified  drift  spread 
by  glacier  waters. 

416 


THE   QUATERNARY  417 

The  materials  of  the  drift  are  in  any  given  place  in  part  unlike 
the  rock  on  which  it  rests.  They  cannot  be  derived  from  the 
underlying  rock  by  weathering,  but  have  been  brought  from 
elsewhere.  Thus  where  a  region  is  underlain  by  sedimentary 
rocks,  as  is  the  drift-covered  area  from  the  Hudson  Eiver  to 
the  Missouri,  the  drift  contains  not  only  fragments  of  limestone, 
sandstone,  and  shale  of  local  derivation,  but  also  pebbles  of  many 
igneous  and  metamorphic  rocks,  such  as  granites,  gneisses, 


FIG.  352.   Stratified  Drift  overlying  Unstratified  Drift,  Massachusetts 

schists,  dike  rocks,  quartzites,  and  the  quartz  of  mineral  veins, 
whose  nearest  source  is  the  Archean  area  of  Canada  and  the 
states  of  our  northern  border.  The  drift  received  its  name 
when  it  was  supposed  that  the  formation  had  been  drifted  by 
floods  and  icebergs  from  outside  sources,  —  a  theory  long  since 
abandoned. 

The  distribution  also  of  the  drift  points  clearly  to  its  peculiar  origin. 
Within  the  limits  of  the  glaciated  area  it  covers  the  country  without 
regard  to  the  relief,  mantling  with  its  debris  not  only  lowlands  and 
valleys  but  also  highlands  and  mountain  slopes. 


418         THE  ELEMENTS  OF  GEOLOGY 

The  boundary  of  the  drift  is  equally  independent  of  the  relief  of  the 
land,  crossing  hills  and  plains  impartially,  unlike  water-laid  deposits, 
whose  margins,  unless  subsequently  deformed,  are  horizontal.  The 
boundary  of  the  drift  is  strikingly  lobate  also,  bending  outward  in 
broad,  convex  curves,  where  there  are  no  natural  barriers  in  the  topog- 
raphy of  the  country  to  set  it  such  a  limit.  Under  these  conditions  such 
a  lobate  margin  cannot  belong  to  deposits  of  rivers,  lakes,  or  ocean, 
but  is  precisely  that  which  would  mark  the  edge  of  a  continental  glacier 
which  deployed  in  broad  tongues  of  ice. 

The  rock  surface  underlying  the  drift.  Over  much  of  its 
area  the  drift  rests  011  firm,  fresh  rock,  showing  that  both  the 
preglacial  mantle  of  residual  waste  and  the  partially  decomposed 
and  broken  rock  beneath  it  have  been  swept  away.  The  under- 
lying rock,  especially  if  massive,  hard,  and  of  a  fine  grain,  has 
often  been  ground  down  to  a  smooth  surface  and  rubbed  to  a 
polish  as  perfect  as  that  seen  on  the  rock  beside  an  Alpine 
glacier  where  the  ice  has  recently  melted  back.  Frequently 
it  has  been  worn  to  the  smooth,  rounded  hummocks  known 
as  roches  moutonnees,  and  even  rocky  hills  have  been  thus 
smoothed  to  flowing  outlines  like  roches  moutonnees  on  a  gigan- 
tic scale.  The  rock  pavement  beneath  the  drift  is  also  marked 
by  long,  straight,  parallel  scorings,  varying  in  size  from  deep 
grooves  to  fine  striae  as  delicate  as  the  hair  lines  cut  by  an  en- 
graver's needle.  Where  the  rock  is  soft  or  closely  jointed  it 
is  often  shattered  to  a  depth  of  several  feet  beneath  the  drift, 
while  stony  clay  has  been  thrust  in  among  the  fragments  into 
which  the  rock  is  broken. 

In  the  presence  of  these  glaciated  surfaces  we  cannot  doubt 
that  the  area  of  the  drift  has  been  overridden  by  vast  sheets  of 
ice  which,  in  their  steady  flow,  rasped  and  scored  the  rock  bed 
beneath  by  means  of  the  stones  with  which  their  basal  layers 
were  inset,  and  in  places  plucked  and  shattered  it. 

Till.  The  unstratified  portion  of  the  drift  consists  chiefly  of 
sheets  of  dense,  stony  clay  called  till,  which  clearly  are  the 


THE  QUATERNARY  419 

ground  moraines  of  ancient  continental  glaciers.  Till  is  an 
unsorted  mixture  of  materials  of  all  sizes,  from  fine  clay  and 
sand,  gravel,  pebbles,  and  cobblestones,  to  large  bowlders.  The 
stones  of  the  till  are  of  many  kinds,  some  having  been  plucked 
from  the  bed  rock  of  the  locality  where  they  are  found,  and 
others  having  been  brought  from  outside  and  often  distant 
places.  Land  ice  is  the  only  agent  known  which  can  spread 
unstratified  material  in  such  extensive  sheets. 

The  fine  material  of  the  till  comes  from  two  different  sources. 
In  part  it  is  derived  from  old  residual  clays,  which  in  the 
making  had  been  leached  of  the  lime  and  other  soluble  ingredi- 
ents of  the  rock  from  which  they  weathered.  In  part  it  consists 
of  sound  rock  ground  fine ;  a  drop  of  acid  on  fresh,  clayey  till 
often  proves  by  brisk  effervescence  that  the  till  contains  much 
undecayed  limestone  flour.  The  ice  sheet,  therefore,  both  scraped 
up  the  mantle  of  long-weathered  waste  which  covered  the  coun- 
try before  its  coming,  and  also  ground  heavily  upon  the  sound 
rock  underneath,  and  crushed  and  wore  to  rock  flour  the 
fragments  which  it  carried. 

The  color  of  unweathered  till  depends  on  that  of  the  materi- 
als of  which  it  is  composed.  Where  red  sandstones  have  con- 
tributed largely  to  its  making,  as  over  the  Triassic  sandstones 
of  the  eastern  states  and  the  Algonkian  sandstones  about  Lake 
Superior,  the  drift  is  reddish.  When  derived  in  part  from  coaly 
shales,  as  over  many  outcrops  of  the  Pennsylvanian,  it  may 
when  moist  be  almost  black.  Fresh  till  is  normally  a  dull  gray 
or  bluish,  so  largely  is  it  made  up  of  the  grindings  of  unoxidized 
rocks  of  these  common  colors. 

Except  where  composed  chiefly  of  sand  or  coarser  stuff,  unweathered 
till  is  often  exceedingly  dense.  Can  you  suggest  by  what  means  it  has 
been  thus  compacted  ?  Did  the  ice  fields  of  the  Glacial  epoch  bear  heavy 
surface  moraines  like  the  medial  and  lateral  moraines  of  valley  glaciers  ? 
Where  was  the  greater  part  of  the  load  of  these  ice  fields  carried,  judg- 
ing from  what  you  know  of  the  glaciers  of  Greenland  ? 


420         THE  ELEMENTS  OF  GEOLOGY 

Bowlders  of  the  drift.  The  pebbles  and  bowlders  of  the  drift 
are  in  part  stream  gravels,  bowlders  of  weathering,  and  other 
coarse  rock  waste  picked  up  from  the  surface  of  the  country  by 
the  advancing  ice,  and  in  part  are  fragments  plucked  from 
ledges  of  sound  rock  after  the  mantle  of  waste  had  been 
removed.  Many  of  the  stones  of  the  till  are  dressed  as  only 
glacier  ice  can  do ;  their  sharp  edges  have  been  blunted  and 
their  sides  faceted  and  scored. 

We  may  easily  find  all  stages  of  this  process  represented  among  the 
pebbles  of  the  till.  Some  are  little  worn,  even  on  their  edges  ;  some 
are  planed  and  scored  on  one  side  only  ;  while  some  in  their  long  jour- 
ney have  been  ground  down  to  many  facets  and  have  lost  much  of  their 
original  bulk.  Evidently  the  ice  played  fast  and  loose  with  a  stone 

carried  in  its  basal  layers, 
now  holding  it  fast  and 
rubbing  it  against  the  rock 
beneath,  now  loosening  its 
grasp  and  allowing  the 
stone  to  turn. 

Bowlders   of    the    drift 

are    sometimes    found    on 
IG.  353.   A  Drumlm,  Wisconsin 

higher  ground   than  their 

parent  ledges.  Thus  bowlders  have  been  left  on  the  sides  of  Mount 
Katahdin,  Maine,  which  were  plucked  from  limestone  ledges  twelve  miles 
distant  and  three  thousand  feet  lower  than  their  resting  place.  In  other 
cases  stones  have  been  carried  over  mountain  ranges,  as  in  Vermont, 
where  pebbles  of  Burlington  red  sandstone  were  dragged  over  the  Green 
Mountains,  three  thousand  feet  in  height,  and  left  in  the  Connecticut 
valley  sixty  miles  away.  No  other  geological  agent  than  glacier  ice 
could  do  this  work. 

The  bowlders  of  the  drift  are  often  large.  Bowlders  ten  and  twenty 
feet  in  diameter  are  not  uncommon,  and  some  are  known  whose  diam- 
eter exceeds  fifty  feet.  As  a  rule  the  average  size  of  bowlders  decreases 
with  increasing  distance  from  their  sources.  Why  ? 

Till  plains.  The  surface  of  the  drift,  where  left  in  its  initial 
state,  also  displays  clear  proof  of  its  glacial  origin.  Over  large 


THE  QUATERNARY 


421 


areas  it  is  spread  in  level  plains  of  till,  perhaps  bowlder-dotted, 
similar  to  the  plains  of  stony  clay  left  in  Spitzbergen  by  the 
recent  retreat  of  some  of  the  glaciers  of  that  island.  In  places 


FIG.  354.  Map  of  a  Portion  of  a  Dramlin  Area  near  Oswego,  New  York 

the  unstratified  drift  is  heaped  in  hills  of  various  kinds,  which 
we  will  now  describe. 

Drumlins.  Drumlins  are  smooth,  rounded  hills  composed  of 
till,  elliptical  in  base,  and  having  their  longer  axes  parallel  to 
the  movement  of  the  ice  as  shown  by  glacial  scorings.  They 


422  THE  ELEMENTS  OF  GEOLOGY 

crowd  certain  districts  in  central  New  York  and  in  southern 
Wisconsin,  where  they  may  be  counted  by  the  thousands. 
Among  the  numerous  drumlins  about  Boston  is  historic  Bunker 
Hill. 

Drumlins  are  made  of  ground  moraine.  They  were  accumu- 
lated and  given  shape  beneath  the  overriding  ice,  much  as  are 
sand  bars  in  a  river,  or  in  some  instances  were  carved,  like 
roches  moutonnees,  by  an  ice  sheet  out  of  the  till  left  by  an 
earlier  ice  invasion. 

Terminal  moraines.  The  glaciated  area  is  crossed  by  belts  of 
thickened  drift,  often  a  mile  or  two,  and  sometimes  even  ten 


Fi<;.  355.   Terminal  Moraine,  Staten  Island 

miles  and  more,  in  breadth,  which  lie  transverse  to  the  move- 
ment of  the  ice  and  clearly  are  the  terminal-  moraines  of  ancient 
ice  sheets,  marking  either  the  limit  of  their  farthest  advance  or 
pauses  in  their  general  retreat. 

The  surface  of  these  moraines  is  a  jumble  of  elevations  and 
depressions,  which  vary  from  low,  gentle  swells  and  shallow 
sags  to  sharp  hills,  a  hundred  feet  or  so  in  height,  and  deep, 
steep-sided  hollows.  Such  tumultuous  hills  and  hummocks,  set 


THE  QUATERNARY  428 

with  depressions  of  all  shapes,  which  usually  are  without  outlet 
and  are  often  occupied  by  marshes,  ponds,  and  lakes,  surely 
cannot  be  the  work  of  running  water.  The  hills  are  heaps  of 
drift,  lodged  beneath  the  ice  edge  or  piled  along  its  front.  The 
basins  were  left  among  the  tangle  of  morainic  knolls  and  ridges 
(Fig.  105)  as  the  margin  of  the  ice  moved  back  and  forth. 
Some  bowl-shaped  basins  were  made  by  the  melting  of  a  mass  of 
ice  left  behind  by  the  retreating  glacier  and  buried  in  its  debris. 

The  stratified  drift.  Like  modern  glaciers  the  ice  sheets  of 
the  Pleistocene  were  ever  being  converted  into  water  about  their 
margins.  Their  limits  on  the  land  were  the  lines  where  their 
onward  flow  was  just  bal- 
anced by  melting  and  evap- 
oration. On  the  surface  of 
the  ice  along  the  marginal 
zone,  rivulets  no  doubt 
flowed  in  summer,  and  found 
their  way  through  crevasses 
to  the  interior  of  the 
glacier  or  to  the  ground. 
Subglacial  streams,  like 
those  of  the  Malaspina  FIG.  356.  Esker,  New  York 

glacier,  issued  from  tunnels  in  the  ice,  and  water  ran  along  the 
melting  ice  front  as  it  is  seen  to  do  about  the  glacier  tongues 
of  Greenland.  All  these  glacier  waters  flowed  away  down  the 
chief  drainage  channels  in  swollen  rivers  loaded  with  glacial 
waste. 

It  is  not  unexpected  therefore  that  there  are  found,  over  all 
the  country  where  the  melting  ice  retreated,  deposits  made  of 
the  same  materials  as  the  till,  but  sorted  and  stratified  by  run- 
ning water.  Some  of  these  were  deposited  behind  the  ice  front 
in  ice-walled  channels,  some  at  the  edge  of  the  glaciers  by  issu- 
ing streams,  and  others  were  spread  to  long  distances  in  front 
of  the  ice  edge  by  glacial  waters  as  they  flowed  away. 


424 


THE  ELEMENTS  OF  GEOLOGY 


Eskers  are  narrow,  winding  ridges  of  stratified  sand  and 
gravel  whose  general  course  lies  parallel  with  the  movement  of 
the  glacier.  These  ridges,  though  evidently  laid  by  running 
water,  do  not  follow  lines  of  continuous  descent,  but  may  be 


FIG.  357.   Kames,  New  York 

found  to  cross  river  valleys  and  ascend  their  sides.  Hence  the 
streams  by  which  eskers  were  laid  did  not  flow  unconfmed  upon 
the  surface  of  the  ground.  We  may  infer  that  eskers  were 
deposited  in  the  tunnels  and  ice-walled  gorges  of  glacial  streams 
before  they  issued  from  the  ice  front. 

Kames  are  sand  and  gravel  knolls,  associated  for  the  most 
part  with  terminal  moraines,  and  heaped  by  glacial  waters  along 

the  margin  of  the  ice. 

Kame  terraces  are  hum- 

rnocky  embankments  of 

stratified  drift  sometimes 
FIG.  358.    Diagram  illustrating  the  For-    found  in  rugged  regions 

along  the  sides  of  valleys. 

In    these    valleys   long 


mation  of  Kame  Terraces 
i,  glacier  ice ;  t,  t,  terraces 


tongues  of  glacier  ice  lay  slowly  melting.  Glacial  waters  took 
their  way  between  the  edges  of  the  glaciers  and  the  hillside, 
and  here  deposited  sand  and  gravel  in  rude  terraces. 


THE   QUATERNARY  425 

Outwash  plains  are  plains  of  sand  and  gravel  which  frequently 
border  terminal  moraines  on  their  outward  face,  and  were  spread 
evidently  by  outwash  from  the  melting  ice.  Outwash  plains  are 
sometimes  pitted  by  bowl-shaped  basins  where  ice  blocks  were 
left  buried  in  the  sand  by  the  retreating  glacier. 

Valley  trains  are  deposits  of  stratified  drift  with  which  river 
valleys  have  been  aggraded.  Valleys  leading  outward  from  the 
ice  front  were  flooded  by  glacial  waters  and  were  filled  often  to 
great  depths  with  trains  of  stream-swept  drift.  Since  the  disap- 
pearance of  the  ice  these  glacial  flood  plains  have  been  dissected 
by  the  shrunken  rivers  of  recent  times  and  left  on  either  side  the 
valley  in  high  terraces.  Valley  trains  head  in  morainic  plains, 
and  their  material  grows  finer  down  valley  and  coarser  toward 
their  sources.  Their  gradient  is  commonly  greater  than  that  of 
the  present  rivers. 

The  extent  of  the  drift.  The  extent  of  the  drift  of  North 
America  and  its  southern  limits  are  best  seen  in  Figure  359.  Its 
area  is  reckoned  at  about  four  million  square  miles.  The  ice 
fields  which  once  covered  so  much  of  our  continent  were  all 
together  ten  times  as  large  as  the  inland  ice  of  Greenland,  and 
about  equal  to  the  enormous  ice  cap  which  now  covers  the 
antartic  regions. 

The  ice  field  of  Europe  was  much  smaller,  measuring  about 
seven  hundred  and  seventy  thousand  square  miles. 

Centers  of  dispersion.  The  direction  of  the  movement  of  the 
ice  is  recorded  plainly  in  the  scorings  of  the  rock  surface,  in 
the  shapes  of  glaciated  hills,  in  the  axes  of  drumlius  and  eskers, 
and  in  trains  of  bowlders,  when  the  ledges  from  which  they 
were  plucked  can  be  discovered.  In  these  ways  it  has  been 
proved  that  in  North  America  there  were  three  centers  where 
ice  gathered  to  the  greatest  depth,  and  from  which  it  flowed 
in  all  directions  outward.  There  were  thus  three  vast  ice 
fields,  —  one  the  Cordilleran,  which  lay  upon  the  Cordilleras  of 
British  America ;  one  the  Keewatin,  which  flowed  out  from  the 


426 


THE  ELEMENTS  OF  GEOLOGY 


province  of  Keewatin,  west  of  Hudson  Bay ;  and  one  the  Labrador 
ice  field,  whose  center  of  dispersion  was  on  the  highlands  of  the 
peninsula  of  Labrador.  As  shown  in  Figure  359,  the  western  ice 
field  extended  but  a  short  way  beyond  the  eastern  foothills  of 
the  Rocky  Mountains,  where  perhaps  it  met  the  far-traveled  ice 


FIG.  359.   Hypothetical  Map  of  the  Pleistocene  Ice  Sheets  of  North  America 
From  Salisbury's  Glacial  Geology  of  New  Jersey 

from  the  great  central  field.  The  Keewatin  and  the  Labrador 
ice  fields  flowed  farthest  toward  the  south,  and  in  the  Mississippi 
valley  the  one  reached  the  mouth  of  the  Missouri  and  the  other 
nearly  to  the  mouth  of  the  Ohio.  In  Minnesota  and  Wisconsin 
and  northward  they  merged  in  one  vast  field. 


THE  QUATERNARY 


427 


The  thickness  of  the  ice  was  so  great  that  it  buried  the  high- 
est mountains  of  eastern  North  America,  as  is  proved  by  the 
transported  bowlders  which  have  been  found  upon  their  sum- 
mits. If  the  land  then  stood  at  its  present  height  above  sea 
level,  and  if  the  average  slope  of  the  ice  were  no  more  than  ten 
feet  to  the  mile,  —  a  slope  so  gentle  that  the  eye  could  not 
detect  it  and  less  than  half  the  slope  of  the  interior  of  the 
inland  ice  of  Green- 
land, —  the  ice  pla- 
teaus about  Hudson 
Bay  must  have 
reached  a  thickness 
of  at  least  ten 
thousand  feet. 


In  Europe  the 
Scandinavian  plateau 
was  the  chief  center 
of  dispersion.  At  the 
time  of  greatest  glaci- 


FIG.  360.   Hypothetical  Map  of  the  Pleistocene 
Ice  Sheet  of  Europe 


ation  a  continuous  field  of  ice  extended  from  the  Ural  Mountains  to  the 
Atlantic,  where,  off  the  coasts  of  Norway  and  the  British  Isles,  it  met 
the  sea  in  an  unbroken  ice  wall.  On  the  south  it  reached  to  southern 
England,  Belgium,  and  central  Germany,  and  deployed  on  the  eastern 
plains  in  wide  lobes  over  Poland  and  central  Russia  (Fig.  360). 

At  the  same  time  the  Alps  supported  giant  glaciers  many  times  the 
size  of  the  surviving  glaciers  of  to-day,  and  a  piedmont  glacier  covered 
Lhe  plains  of  northern  Switzerland. 

The  thickness  of  the  drift.  The  drift  is  far  from  uniform  hi 
thickness.  It  is  comparatively  thin  and  scanty  over  the  Lauren- 
tian  highlands  and  the  rugged  regions  of  New  England,  while 
from  southern  New  York  and  Ontario  westward  over  the  Mis- 
sissippi valley,  and  on  the  great  western  plains  of  Canada,  it 
exceeds  an  average  of  one  hundred  feet  over  wide  areas,  and  in 
places  has  five  and  six  times  that  thickness.  It  was  to  this 


428  THE  ELEMENTS  OF  GEOLOGY 

marginal  belt  that  the  ice  sheets  brought  their  loads,  while 
northwards,  nearer  the  centers  of  dispersion,  erosion  was  exces- 
sive and  deposition  slight. 

Successive  ice  invasions  and  their  drift  sheets.  Recent  stud- 
ies of  the  drift  prove  that  it  does  not  consist  of  one  indivis- 
ible formation,  but  includes  a  number  of  distinct  drift  sheets, 
each  with  its  own  peculiar  features.  The  Pleistocene  epoch 
consisted,  therefore,  of  several  glacial  stages, —  during  each  of 
which  the  ice  advanced  far  southward,  —  together  with  the 
intervening  interglacial  stages  when,  under  a  milder  climate, 
the  ice  melted  back  toward  its  sources  or  wholly  disappeared. 

The  evidences  of  such  interglacial  stages,  and  the  means  by  which 
the  different  drift  sheets  are  told  apart,  are  illustrated  in  Figure  361. 
Here  the  country  from  N  to  S  is  wholly  covered  by  drift,  but  the  drift 


FIG.  861.   Diagram  illustrating  Criteria  by  which  Different 
Drift  Sheets  are  distinguished 

from  N  to  m  is  so  unlike  that  from  m  to  S  that  we  may  believe  it  the 
product  of  a  distinct  ice  invasion  and  deposited  during  another  and  far 
later  glacial  stage.  The  former  drift  is  very  young,  for  its  drainage  is 
as  yet  immature,  and  there  are  many  lakes  and  marshes  upon  its  sur- 
face ;  the  latter  is  far  older,  for  its  surface  has  been  thoroughly  dissected 
by  its  streams.  The  former  is  but  slightly  weathered,  while  the  latter 
is  so  old  that  it  is  deeply  reddened  by  oxidation  and  is  leached  of  its 
soluble  ingredients  such  as  lime.  The  younger  drift  is  bordered  by  a 
distinct  terminal  moraine,  while  the  margin  of  the  older  drift  is  not 
thus  marked.  Moreover,  the  two  drift  sheets  are  somewhat  unlike  in 
composition,  and  the  different  proportion  of  pebbles  of  the  various 
kinds  of  rocks  which  they  contain  shows  that  their  respective  glaciers 
followed  different  tracks  and  gathered  their  loads  from  different  regions. 
Again,  in  places  beneath  the  younger  drift  there  is  found  the  buried 
land  surface  of  an  older  drift  with  old  soils,  forest  grounds,  and  vege- 
table deposits,  containing  the  remains  of  animals  and  plants,  which 
tell  of  the  climate  of  the  interglacial  stage  in  which  they  lived. 


THE  QUATERNARY  429 

By  such  differences  as  these  the  following  drift  sheets  have 
been  made  out  in  America,  and  similar  subdivisions  have  been 
recognized  in  Europe. 

5  The  Wisconsin  formation 
4  The  lowan  formation 
3  The  Illinoian  formation 
2  The  Kansan  formation 
1  The  Xebraskan  formation 

The  earliest  glacial  stage  is  recorded  in  an  exceedingly  old 
drift  sheet  found  in  eastern  Nebraska  and  other  states  and 
parted  from  the  overlying  Kansan  by  the  deposits  of  the  first 
interglacial  epoch,  called  the  Aftonian.  Quite  generally  pre- 
Kansan  drift  has  been  swept  away  by  later  ice  invasions,  but 
in  places  it  is  found  deeply  buried  beneath  their  ground  moraines. 

The  two  succeeding  stages  mark  the  greatest  snowfall  of  the 
Glacial  epoch.  In  Kansan  times  the  Keewatin  ice  field  slowly 
grew  southward  until  it  reached  fifteen  hundred  miles  from  its 
center  of  dispersion  and  extended  from  the  Arctic  Ocean  to 
northeastern  Kansas.  In  the  Illinoian  stage  the  Labrador  ice 
field  stretched  from  Hudson  Straits  nearly  to  the  Ohio  Eiver  in 
Illinois.  In  the  lowan  and  the  Wisconsin,  the  closing  stages  of 
the  Glacial  epoch,  the  readvancing  ice  fields  fell  far  short  of  their 
former  limits  in  the  Mississippi  valley,  but  in  the  eastern  states 
the  Labrador  ice  field  during  Wisconsin  times  overrode  for  the 
most  part  all  earlier  deposits,  and,  covering  New  England,  prob- 
ably met  the  ocean  in  a  continuous  wall  of  ice  which  set  its  bergs 
afloat  from  Massachusetts  to  northern  Labrador. 

We  select  for  detailed  description  the  Kansan  and  the  Wis- 
consin formations  as  representatives,  the  one  of  the  older  and 
the  other  of  the  younger  drift  sheets; 

The  Kansan  formation.  The  Kansan  drift  consists  for  the 
most  part  of  a  sheet  of  clayey  till  carrying  smaller  bowlders 
than  the  later  drift.  Few  traces  of  drumlins,  kames,  or  terminal 


430 


THE  QUATERNARY 


431 


moraines  are  found  upon  the  Kansan  drift,  and  where  thick 
enough  to  mask  the  preexisting  surface,  it  seems  to  have  been 
spread  originally  in  level  plains  of  tilL 

The  initial  Kansan  plain  has  been  worn  by  running  water 
until  there  are  now  left  only  isolated  patches  and  the  narrow 
strips  and  crests  of  the  divides,  which  still  rise  to  the  ancient 
leveL  The  valleys  of  the  larger  streams  have  been  opened  wide. 
Their  well-developed  tributaries  have  carved  nearly  the  entire 
plain  to  valley  slopes  (Figs.  50  B,  and  59  ).  The  lakes  and  marshes 
which  once  marked  the  infancy  of  the  region  have  long  since 


FIG.  363.   Plain  of  Wisconsin  Drift,  Iowa 

been  effaced  The  drift  is  also  deeply  weathered.  The  till,  origi- 
nally blue  in  color,  has  been  yellowed  by  oxidation  to  a  depth 
of  ten  and  twenty  feet  and  even  more,  and  its  surface  is  some- 
times rusted  to  terra-cotta  red.  To  a  somewhat  less  depth  it  has 
been  leached  of  its  lime  and  other  soluble  ingredients.  In  the 
weathered  zone  its  pebbles,  especially  where  the  till  is  loose  in 
texture,  are  sometimes  so  rotted  that  granites  may  be  crumbled 
with  the  fingers.  The  Kansan  drift  is  therefore  old. 

The  Wisconsin  formation.  The  Wisconsin  drift  sheet  is  but 
little  weathered  and  eroded,  and  therefore  is  extremely  young. 
Oxidation  has  effected  it  but  slightly,  and  lime  and  other 


432  THE  ELEMENTS  OF  GEOLOGY 

soluble  plant  foods  remain  undissolved  even  at  the  grass  roots. 
Its  river  systems  are  still  in  their  infancy  (Fig.  50,  A).  Swamps 
and  peat  bogs  are  abundant  on  its  undrained  surface,  and  to 
this  drift  sheet  belong  the  lake  lands  of  our  northern  states 
and  of  the  Laurentian  peneplain  of  Canada. 

The  lake  basins  of  the  Wisconsin  drift  are  of  several  different 
classes.  Many  are  shallow  sags  in  the  ground  moraine.  Still  more 
numerous  are  the  lakes  set  in  hollows  among  the  hills  of  the  terminal 
moraines  ;  such  as  the  thousands  of  lakelets  of  eastern  Massachusetts. 
Indeed,  the  terminal  moraines  of  the  Wisconsin  drift  may  often  be 
roughly  traced  on  maps  by  means  of  belts  of  lakes  and  ponds.  Some 
lakes  are  due  to  the  blockade  of  ancient  valleys  by  morainic  debris, 
and  this  class  includes  many  of  the  lakes  of  the  Adirondacks,  the 
mountain  regions  of  New  England,  and  the  Laurentian  area.  Still 
other  lakes  rest  in  rock  basins  scooped  out  by  glaciers.  In  many  cases 
lakes  are  due  to  more  than  one  cause,  as  where  preglacial  valleys  have 
both  been  basined  by  the  ice  and  blockaded  by  its  moraines.  The  Finger 
lakes  of  New  York,  for  example,  occupy,  such  glacial  troughs. 

Massive  terminal  moraines,  which  mark  the  farthest  limits  to 
which  the  Wisconsin  ice  advanced,  have  been  traced  from  Cape 
Cod  and  the  islands  south  of  New  England,  across  the  Appala- 
chians and  the  Mississippi  valley,  through  the  Dakotas,  and  far 
to  the  north  over  the  plains  of  British  America.  Where  the  ice 
halted  for  a  time  in  its  general  retreat,  it  left  recessional  mo- 
raines, as  this  variety  of  the  terminal  moraine  is  called.  The 
moraines  of  the  Wisconsin  drift  lie  upon  the  country  like  great 
festoons,  each  series  of  concentric  loops  marking  the  utmost 
advance  of  broad  lobes  of  the  ice  margin  and  the  various  pauses 
in  their  recession. 

Behind  the  terminal  moraines  lie  wide  till  plains,  in  places 
studded  thickly  with  drumlins,  or  ridged  with  an  occasional 
esker.  Great  outwash  plains  of  sand  and  gravel  lie  in  front  of 
the  moraine  belts,  and  long  valley  trains  of  coarse  gravels  tell 
of  the  swift  and  powerful  rivers  of  the  time. 


THE   QUATERNARY 


433 


The  loess  of  the  Mississippi  valley.  A  yellow  earth,  quite 
like  the  loess  of  China,  is  laid  broadly  as  a  surface  deposit  over 
the  Mississippi  valley  from  eastern  Nebraska  to  Ohio  outside 
the  boundaries  of  the  lowan  and  the  Wisconsin  drift.  Much 
of  the  loess  was  deposited  in  lowan  times.  It  is  younger  than 
the  earlier  drift  sheets,  for  it  overlies  their  weathered  and  eroded 
surfaces.  It  thickens  to  the  lowan  drift  border,  but  is  not  found 
upon  that  drift.  It  is  older  than  the  Wisconsin,  for  in  many 
places  it  passes  underneath  the  Wisconsin  terminal  moraines. 


FIG.  364.    Bank  of  Loess,  Iowa 

In  part  the  loess  seems  to  have  been  washed  from  glacial  waste 
and  spread  in  sluggish  glacial  waters,  and  in  part  to  have  been 
distributed  by  the  wind  from  plains  of  aggrading  glacial  streams. 
The  effects  of  the  ice  invasions  on  rivers.  The  repeated  ice 
invasions  of  the  Pleistocene  profoundly  disarranged  the  drainage 
systems  of  our  northern  states.  In  some  regions  the  ancient 
valleys  were  completely  filled  with  drift.  On  the  withdrawal 
of  the  ice  the  streams  were  compelled  to  find  their  way,  as  best 
they  could,  over  a  fresh  land  surface,  where  we  now  find  them 
flowing  on  the  drift  in  young,  narrow  channels.  But  hundreds  of 


434 


THE  ELEMENTS  OF  GEOLOGY 


feet  below  the  ground  the  well  driller  and  the  prospector  for  coal 
and  oil  discover  deep,  wide,  buried  valleys  cut  in  rock,  —  the 
channels  of  preglacial  and  interglacial  streams.  In  places  the 
ancient  valleys  were  filled  with  drift  to  a  depth  of  a  hundred 

feet,  and  sometimes  even  to 
a  depth  of  four  hundred  and 
five  hundred  feet.  In  such 
valleys,  rivers  now  flow  high 
above  their  ancient  beds  of 
rock  on  floors  of  valley  drift. 
Many  of  the  valleys  of  our 
present  rivers  are  but  patch- 
works of  preglacial,  inter- 
glacial,  and  postglacial 
courses  (Fig.  &66).  Here 
the  river  winds  along  an 
ancient  valley  with  gently 
sloping  sides  and  a  wide 
alluvial  floor  perhaps  a  mile 
or  so  in  width,  and  there  it 
enters  a  young,  rock-walled 
gorge,  whose  rocky  bed  may 
be  crossed  by  ledges  over 
which  the  river  plunges  in 
waterfalls  and  rapids. 

In  such  cases  it  is  possible 
that  the  river  was  pushed  to 
one  side  of  its  former  valley 
by  a  lobe  of  ice,  and  com- 
pelled to  cut  a  new  channel  in  the  adjacent  uplands.  A  section 
of  the  valley  may  have  been  blockaded  with  morainic  waste, 
and  the  lake  formed  behind  the  barrier  may  have  found  outlet 
over  the  country  to  one  side  of  the  ancient  drift-filled  valley. 
In  some  instances  it  would  seem  that  during  the  waning  of  the 


N.Mai'Ws¥lIe~(fSw.VA.~j    ^     MARYLAND 


FIG.  365.   Preglacial  Drainage,  Upper 

Ohio  Valley 
After  Chamberlin  and  Leverett 


THE  QUATERNARY  435 

ice  sheets,  glacial  streams,  while  confined  within  walls  of  stag- 
nant ice,  cut  down  through  the  ice  and  incised  their  channels 
on  the  underlying  country,  in  some  cases  being  let  down  on  old 
river  courses,  and  in  other  cases  excavating  gorges  in  adjacent 
uplands. 

Pleistocene  lakes.  Temporary  lakes  were  formed  wherever 
the  ice  front  dammed  the  natural  drainage  of  the  region.  Some, 
held  in  the  minor  valleys 
crossed  by  ice  lobes,  were 
small,  and  no  doubt  many 
were  too  short-lived  to  leave 
lasting  records.  Others,  Xv  ~~~~~ 

long  held  against  the  north- 
ward sloping  country  by  the  FIG.  366.  A  Patchwork  Valley 
retreating   ice   edge,  left    in     a  and  a',  ancient  courses  still  occupied  by 
their    beaches,   their    clayey          the  river ;  6  po^glacial  gorge ;  c,  ancient 
J    J           course  now  filled  with  drift 

beds,  and  their  outlet  chan- 
nels permanent  evidences  of  their  area  and  depth.    Some  of 
these  glacial  lakes  are  thus  known  to  have  been  larger  than  any 
present  lake. 

Lake  Agassiz,  named  in  honor  of  the  author  of  the  theory  of  conti- 
nental glaciation,  is  supposed  to  have  been  held  by  the  united  front  of 
the  Keewatin  and  the  Labrador  ice  fields  as  they  finally  retreated  down 
the  valley  of  the  Red  River  of  the  North  and  the  drainage  basin  of 
Lake  Winnipeg.  From  first  to  last  Lake  Agassiz  covered  a  hundred  and 
ten  thousand  square  miles  in  Manitoba  and  the  adjacent  parts  of  Min- 
nesota and  North  Dakota,  —  an  area  larger  than  all  the  Great  Lakes 
combined.  It  discharged  its  waters  across  the  divide  which  held  it  on 
the  south,  and  thus  excavated  the  valley  of  the  Minnesota  River.  The 
lake  bed  —  a  plain  of  till  —  was  spread  smooth  and  level  as  a  floor  with 
lacustrine  silts.  Since  Lake  Agassiz  vanished  with  the  melting  back 
of  the  ice  beyond  the  outlet  by  the  Nelson  River  into  Hudson  Bay, 
there  has  gathered  on  its  floor  a  deep  humus,  rich  in  the  nitrogenous 
elements  so  needful  for  the  growth  of  plants,  and  it  is  to  this  soil  that 
the  region  owes  its  well-known  fertility. 


436         THE  ELEMENTS  OF  GEOLOGY 

The  Great  Lakes.  The  basins  of  the  Great  Lakes  are  broad 
preglacial  river  valleys,  warped  by  movements  of  the  crust  still 
in  progress,  enlarged  by  the  erosive  action  of  lobes  of  the  con- 
tinental ice  sheets,  and  blockaded  by  their  drift.  The  compli cated 
glacial  and  postglacial  history  of  the  lakes  is  recorded  in  old 
strand  lines  which  have  been  traced  at  various  heights  about 
them,  showing  their  areas  and  the  levels  at  which  their  waters 
stood  at  different  times. 

With  the  retreat  of  the  lobate  Wisconsin  ice  sheet  toward 
the  north  and  east,  the  southern  and  western  ends  of  the  basins 
of  the  Great  Lakes  were  uncovered  first ;  and  here,  between  the 
receding  ice  front  and  the  slopes  of  land  which  faced  it,  lakes 
gathered  which  increased  constantly  in  size. 

The  lake  which  thus  came  to  occupy  the  western  end  of  the 
Lake  Superior  basin  discharged  over  the  divide  at  Duluth  down 
the  St.  Croix  River,  as  an  old  outlet  channel  proves ;  that  which 
held  the  southern  end  of  the  basin  of  Lake  Michigan  sent  its 
overflow  across  the  divide  at  Chicago  via  the  Illinois  River  to 
the  Mississippi ;  the  lake  which  covered  the  lowlands  about  the 
western  end  of  Lake  Erie  discharged  its  waters  at  Fort  Wayne 
into  the  Wabash  River. 

The  ice  still  blocked  the  Mohawk  and  St.  Lawrence  valleys 
on  the  east,  while  on  the  west  it  had  retreated  far  to  the  north. 
The  lakes  become  confluent  in  wide  expanses  of  water,  whose 
depths  and  margins,  as  shown  by  their  old  lake  beaches,  varied 
at  different  times  with  the  position  of  the  confining  ice  and 
with  warpings  of  the  land.  These  vast  water  bodies,  which  at 
one  or  more  periods  were  greater  than  all  the  Great  Lakes  com- 
bined, discharged  at  various  times  across  the  divide  at  Chicago, 
near  Syracuse,  New  York,  down  the  Mohawk  valley,  and  by  a 
channel  from  Georgian  Bay  into  the  Ottawa  River.  Last  of  all 
the  present  outlet  by  the  St.  Lawrence  was  established. 

The  beaches  of  the  glacial  lakes  just  mentioned  are  now  far  from 
horizontal.  That  of  the  lake  which  occupied  the  Ontario  basin  has  an 


THE  QUATERNARY  437 

elevation  of  three  hundred  and  sixty-two  feet  above  tide  at  the  west  and 
of  six  hundred  and  seventy-five  feet  at  the  northeast,  proving  here  a 
differential  movement  of  the  land  since  glacial  times  amounting  to 
more  than  three  hundred  feet.  The  beaches  which  mark  the  successive 
heights  of  these  glacial  lakes  are  not  parallel ;  hence  the  warping  began 
before  the  Glacial  epoch  closed.  We  have  already  seen  that  the  canting 
of  the  region  is  still  in  progress  (p.  198). 

The  Champlain  subsidence.  As  the  Glacial  epoch  approached 
its  end,  and  the  Labrador  ice  field  melted  back  for  the  last  time 
to  near  its  source,  the  land  on  which  the  ice  had  lain  in  eastern 
North  America  was  so  depressed  that  the  sea  now  spread  far  and 
wide  up  the  St.  Lawrence  valley.  It  joined  with  Lake  Ontario, 
and  extending  down  the  Champlain  and  Hudson  valleys,  made 
an  island  of  New  England  and  the  maritime  provinces  of  Canada. 

The  proofs  of  this  subsidence  are  found  in  old  sea  beaches 
and  sea-laid  clays  resting  on  Wisconsin  till.  At  Montreal  such 
terraces  are  found  six  hundred  and  twenty  feet  above  sea  level, 
and  along  Lake  Champlain  —  where  the  skeleton  of  a  whale 
was  once  found  among  them  —  at  from  five  hundred  to  four 
hundred  feet.  The  heavy  delta  which  the  Mohawk  Eiver  built 
at  its  mouth  in  this  arm  of  the  sea  now  stands  something  more 
than  three  hundred  feet  above  sea  level.  The  clays  of  the  Cham- 
plain  subsidence  pass  under  water  near  the  mouth  of  the  Hud- 
son, and  in  northern  New  Jersey  they  occur  two  hundred  feet 
below  tide.  In  these  elevations  we  have  measures  of  the  warp- 
ing of  the  region  since  glacial  times. 

The  western  United  States  in  glacial  times.  The  western 
United  States  was  not  covered  during  the  Pleistocene  by  any 
general  ice  sheet,  but  all  the  high  ranges  were  capped  with 
permanent  snow  and  nourished  valley  glaciers,  often  many  times 
the  size  of  the  existing  glaciers  of  the  Alps.  In  almost  every 
valley  of  the  Sierras  and  the  Eockies  the  records  of  these  van- 
ished ice  streams  may  be  found  in  cirques,  glacial  troughs,  roches 
moutonne*es,  and  morainic  deposits. 


438         THE  ELEMENTS  OF  GEOLOGY 

It  was  during  the  Glacial  epoch  that  Lakes  Bonneville  and 
Lahontan  (p.  107)  were  established  in  the  Great  Basin,  whose 
climate  must  then  have  been  much  more  moist  than  now. 

The  driftless  area.  In  the  upper  Mississippi  valley  there  is  an  area 
of  about  ten  thousand  square  miles  in  southwestern  Wisconsin  and  the 
adjacent  parts  of  Iowa  and  Minnesota,  which  escaped  the  ice  invasions. 
The  rocks  are  covered  with  residual  clays,  the  product  of  long  pre- 
glacial  weathering.  The  region  is  an  ancient  peneplain,  uplifted  and 
dissected  in  late  Tertiary  times,  with  mature  valleys  whose  gentle 


FIG.  367.   A  Valley  in  the  Driftless  Area 

gradients  are  unbroken  by  waterfalls  and  rapids.  Thus  the  driftless 
area  is  in  strong  contrast  with  the  immature  drift  topography  about  it, 
where  lakes  and  waterfalls  are  common.  It  is  a  bit  of  preglacial  land- 
scape, showing  the  condition  of  the  entire  region  before  the  Glacial 
epoch. 

The  driftless  area  lay  to  one  side  of  the  main  track  of  both  the  Kee- 
watin  and  the  Labrador  ice  fields,  and  at  the  north  it  was  protected  by 
the  upland  south  of  Lake  Superior,  which  weakened  and  retarded  the 
movement  of  the  ice. 

South  of  the  driftless  area  the  Mississippi  valley  was  invaded  at  dif- 
ferent times  by  ice  sheets  from  the  west,  —  the  Kansan  and  the  lowan,  — 
and  again  by  the  Illinoian  ice  sheet  from  the  east.  Again  and  again  the 


THE  QUATERNARY 


439 


Mississippi  River  was  pushed  to  one  side  or  the  other  of  its  path.  The 
ancient  channel  which  it  held  along  the  Illinoian  ice  front  has  been 
traced  through  southeastern  Iowa  for  many  miles. 

Benefits  of  glaciation.  Like  the  driftless  area,  the  preglacial 
surface  over  which  the  ice  advanced  seems  to  have  been  well 
dissected  after  the  late  Tertiary  uplifts,  and  to  have  been  carved 
in  many  places  to  steep  valley  slopes  and  rugged  hills.  The 
retreating  ice  sheets,  which  left  smooth  plains  and  gently  rolling 
country  over  the  wide  belt  where  glacial  deposition  exceeded 


FIG.  368.   Cross  Section  of  a  Valley  in  Eastern  Iowa 

a,  country  rock ;  b,  Kansan  till ;  c,  loess ;  t,  terrace  of  reddish  sands  and  decayed 
pebbles  above  reach  of  present  stream ;  s,  stream ;  fp,  flood  plain  of  s.  What 
is  the  age  of  rock-cut  valley  and  of  the  alluvium  which  partially  fills  it,  com- 
pared with  that  of  the  Kansan  till  ?  with  that  of  the  loess  ?  Give  the  complete 
history  recorded  in  the  section. 

glacial  erosion,  have  made  travel  and  transportation  easier  than 
they  otherwise  would  have  been. 

The  preglacial  subsoils  were  residual  clays  and  sands,  com- 
posed of  the  insoluble  elements  of  the  country  rock  of  the  local- 
ity, with  some  minglings  of  its  soluble  parts  still  undissolved. 
The  glacial  subsoils  are  made  of  rocks  of  many  kinds,  still  un- 
decayed  and  largely  ground  to  powder.  They  thus  contain  an 
inexhaustible  store  of  the  mineral  foods  of  plants,  and  in  a  form 
made  easily  ready  for  plant  use. 

On  the  preglacial  hillsides  the  humus  layer  must  have  been 
comparatively  thin,  while  the  broad  glacial  plains  have  gathered 


440  THE  ELEMENTS   OF  GEOLOGY 

deep  black  soils,  rich  in  carbon  and  nitrogen  taken  from  the 
atmosphere.  To  these  soils  and  subsoils  a  large  part  of  the 
wealth  and  prosperity  of  the  glaciated  regions  of  our  country 
must  be  attributed. 

The  ice  invasions  have  also  added  very  largely  to  the  water 
power  of  the  country.  The  rivers  which  in  preglacial  times 
were  flowing  over  graded  courses  for  the  most  part,  were  pushed 
from  their  old  valleys  and  set  to  flow  on  higher  levels,  where 
they  have  developed  waterfalls  and  rapids.  This  power  will 
probably  be  fully  utilized  long  before  the  coal  beds  of  the  coun- 
try are  exhausted,  and  will  become  one  of  the  chief  sources  of 
the  national  wealth. 

The  Recent  epoch.  The  deposits  laid  since  glacial  times 
graduate  into  those  now  forming  along  the  ocean  shores,  on 
lake  beds,  and  in  river  valleys.  Slow  and  comparatively  slight 
changes,  such  as  the  warpings  of  the  region  of  the  Great  Lakes, 
have  brought  about  the  geographical  conditions  of  the  present. 
The  physical  history  of  the  Eecent  epoch  needs  here  no  special 
mention. 

THE  LIFE  OF  THE  QUATERNARY 

During  the  entire  Quaternary,  invertebrates  and  plants  suf- 
fered little  change  in  species,  —  so  slowly  are  these  ancient  and 
comparatively  simple  organisms  modified.  The  Mammalia,  on  the 
other  hand,  have  changed  much  since  the  beginning  of  Quater- 
nary time :  the  various  species  of  the  present  have  been  evolved, 
and  some  lines  have  become  extinct.  These  highly  organized 
vertebrates  are  evidently  less  stable  than  are  lower  types  of  ani- 
mals, and  respond  more  rapidly  to  changes  in  the  environment. 

Pleistocene  mammals.  In  the  Pleistocene  the  Mammalia 
reached  their  culmination  both  in  size  and  in  variety  of  forms, 
and  were  superior  in  both  these  respects  to  the  mammals  of 
to-day.  In  Pleistocene  times  in  North  America  there  were  sev- 
eral species  of  bison,  —  one  whose  widespreading  horns  were 


THE  QUATERNARY 


441 


ten  feet  from  tip  to  tip,  —  a  gigantic  moose  elk,  a  giant  rodent 
(Castoroides)  five  feet  long,  several  species  of  musk  oxen, 
several  species  of  horses,  —  more  akin,  however,  to  zebras  than 


FIG.  369.   Megatherium 

to  the  modern  horse,  —  a  huge  lion,  several  saber-tooth  tigers, 
immense  edentates  of  several  genera,  and  largest  of  all  the 
mastodon  and  mammoth. 

The  largest  of  the  edentates  was  the  Megatherium,  a  clumsy  ground 
sloth  bigger  than  a  rhinoceros.  The  bones  of  the  Megatherium  are 
extraordinarily  massive,  —  the  thigh  bone  being  thrice  as  thick  as  that 
of  an  elephant," — and  the  animal  seems  to  have  been  well  able  to  get  its 


FIG.  370.   Glyptodon 

living  by  overthrowing  trees  and  stripping  off  their  leaves.  The  Glyp- 
todon was  a  mailed  edentate,  eight  feet  long,  resembling  the  little  arma- 
dillo! These  edentates  survived  from  Tertiary  times,  and  in  the  warmer 
stages  of  the  Pleistocene  ranged  north  as  far  as  Ohio  and  Oregon. 


442 


THE  ELEMENTS  OF  GEOLOGY 


The  great  proboscidians  of  the  Glacial  epoch  were  about  the 
size  of  modern  elephants,  and  somewhat  smaller  than  their 
ancestral  species  in  the  Pliocene.  The  Mastodon  ranged  over  all 
North  America  south  of  Hudson  Bay,  but  had  become  extinct 
in  the  Old  World  at  the  end  of  the  Tertiary.  The  elephants 
were  represented  by  the  Mammoth,  which  roamed  in  immense 
herds  from  our  middle  states  to  Alaska,  and  from  Arctic  Asia 
to  the  Mediterranean  and  Atlantic. 

It  is  an  oft-told  story  how  about  a  century  ago,  near  the  Lena 
River  in  Siberia,  there  was  found  the  body  of  a  mammoth  which  had 
been  safely  preserved  in  ice  for  thousands  of  years,  how  the  flesh  was 
eaten  by  dogs  and  bears,  and  how  the  eyes  and  hoofs  and  portions 
of  the  hide  were  taken  with  the  skeleton  to  St.  Petersburg.  Since 
then  several  other  carcasses  of  the  mammoth,  similarly  preserved  in 
ice,  have  been  found  in  the  same  region,  —  one  as  recently  as  1901. 
We  know  from  these  remains  that  the  animal  was  clothed  in  a  coat  of 
long,  coarse  hair,  with  thick  brown  fur  beneath. 

• 
The  distribution  of  animals  and  plants.    The  distribution  of 

species  in  the  Glacial  epoch  was  far  different  from  that  of  the 

present.  In  the  glacial 
stages  arctic  species 
ranged  south  into  what 
are  now  temperate  lati- 
tudes.  The  walrus 
throve  along  the  shores 
of  Virginia  and  the 
musk  ox  grazed  in  Iowa 
and  Kentucky.  In 

FIG.  371.  Skull  of  Musk  Ox,  from  Pleisto-  Europe  the  reindeer  and 
cene  Deposits,  Iowa  arcfcic  fox  reacned  the 

Pyrenees.  During  the  Champlain  depression  arctic  shells  lived 
along  the  shore  of  the  arm  of  the  sea  which  covered  the  St. 
Lawrence  valley.  In  interglacial  times  of  milder  climate  the 
arctic  fauna-flora  retreated,  and  their  places  were  taken  by  plants 


THE  QUATERNARY  443 

and  animals  from  the  south.  Peccaries,  now  found  in  Texas, 
ranged  into  Michigan  and  New  York,  while  great  sloths  from 
South  America  reached  the  middle  states.  Interglacial  beds  at 
Toronto,  Canada,  contain  remains  of  forests  of  maple,  elm,  and 
papaw,  with  mollusks  now  living  in  the  Mississippi  basin. 

What  changes  in  the  forests  of  your  region  Would  be  brought  about, 
and  in  what  way,  if  the  climate  should  very  gradually  grow  colder? 
What  changes  if  it  should  grow  warmer  ? 

On  the  Alps  and  the  highest  summits  of  the  White  Mountains  of 
New  England  are  found  colonies  of  arctic  species  of  plants  and  insects. 
How  did  they  come  to  be  thus  separated  from  their  home  beyond  the 
arctic  circle  by  a  thousand  miles  and  more  of  temperate  climate 
impossible  to  cross? 

Man.  Along  with  the  remains  of  the  characteristic  animals 
of  the  time  which  are  now  extinct  there  have  been  found  in 
deposits  of  the  Glacial  epoch  in  the  Old  World  relics  of  Pleisto- 
cene Man,  his  bones,  and  articles  of  his  manufacture.  In  Europe, 
where  they  have  best  been  studied,  human  relics  occur  chiefly 
in  peat  bogs,  in  loess,  in  caverns  where  man  made  his  home, 
and  in  high  river  terraces  sometimes  eighty  and  a  hundred  feet 
above  the  present  flood  plains  of  the  streams. 

In  order  to  understand  the  development  of  early  man5  we 
should  know  that  prehistoric  peoples  are  ranked  according  to 
the  materials  of  which  their  tools  were  made  and  the  skill 
shown  in  their  manufacture.  There  are  thus  five  well-marked 
stages  of  human  culture  preceding  the  written  annals  of  history. 

The  Eolithic  stage  (the  Dawn  of  the  Stone  Age)  is  attested 
only  by  chipped  flints,  so  crudely  worked  that  there  has  been 
much  doubt  whether  they  may  not  have  been  produced  by 
natural  processes,  such  as  frost,  the  impact  of  pebbles  in  swift 
torrents,  and  the  pressure  of  overlying  beds. 

This  stage  may  have  been  preceded  by  an  earlier,  in  which 
man's  only  tools  and  weapons  were  sticks  of  wood,  rock  splinters, 
and  chance  pebbles  of  brook  or  beach. 


444 


THE  ELEMENTS  OF  GEOLOGY 


In  the  Paleolithic  stage  (the  Old  Stone  Age)  the  first  imple- 
ments were  nodules  of  flint  chipped  rudely  to  sharp  edges  and 
held  unhafted  in  the  hand  as  a  general-purpose  tool  (Fig.  372). 
With  the  progress  of  thousands  of  years  these  handstones  were 
much  improved,  and  flint  flakes  were  retouched  to  a  variety  of 

characteristic  forms  .adapted 
to  many  different  uses. 

The  Neolithic  stage  (the 
New  Stone  Age)  witnessed 
the  introduction  of  weapons 
and  tools  of  polished  stone. 
To  this  stage  belonged  the 
North  American  Indian  at 
the  time  of  the  discovery  of 
the  continent. 

The  Bronze  stage  and  the 
Iron  stage  were  marked  by 
the  introduction  of  these 
metals. 

Pithecanthropus  (ape  man) 
erectus.  The  oldest  remains 

of  man  are  parts  of  a  skeleton  found  in  Java  in  river  deposits 
late  Pliocene  or  very  early  Pleistocene  in  age.  Their  antiquity 
is  therefore  to  be  measured  in  hundreds  of  thousands  of  years. 
These  parts  comprise  (1)  the  skullcap,  with  a  retreating  fore- 
head as  low  as  a  chimpanzee's,  and  with  beetling  brow  ridges 
of  ferocious  aspect ;  (2)  three  molar  teeth  somewhat  coarser 
than  man's  to-day ;  and  (3)  a  thigh  bone  whose  articulations 
indicate  an  erect  posture  and  whose  length  proves  a  stature 
about  equal  to  that  of  the  average  Anglo-Saxon.  The  cranial 
capacity,  estimated  at  about  900  cubic  centimeters,  is  well 
above  the  ape's  (which  never  exceeds  600  cubic  centimeters), 
although  it  is  much  less  than  that  of  the  lowest  of  existing 
human  races.  The  estimated  ratio  of  brain  to  body  weight 


FIG.  372.    Paleolithic  Implement  from 
Great  Britain 


THE  QUATERNARY  445 

(1 :  94)  is  also  intermediate  between  that  of  the  ape  (1 :  183) 
and  that  of  man  (1:51).  The  low  and  narrow  frontal  area 
of  the  brain,  the  seat  of  the  higher  mental  faculties,  the  control 
of  conduct,  and  the  registration  of  experience,  indicates  that  the 
Javau  ape  man  was  a  creature  of  feeble  intelligence  and  little 
wisdom.  The  brain  area  of  motor  speech  is  twice  that  of  the 
highest  ape,  although  but  one-half  that  of  present  man,  and 
thus  permits  the  inference  that  Pithecanthropus  may  have  pos- 
sessed the  rudiments  of  language.  Certainly  he  had  attained 
a  stage  far  higher  than  any  of  the  primates  below  man. 
Arboreal  life  had  been  abandoned  ;  erect  posture  freed  the  fore 
limbs  from  the  burden  of  the  support  of  the  body  and  from  the 
work  of  locomotion,  and  gave  the  advantage  of  agile  movements 
of  defense  and  offense  ;  and  the  large  increase  of  brain  was  no 
doubt  accompanied  by  a  higher  mentality,  able  to  discern  rela- 
tions which  no  ape  can  grasp. 

The  ancestors  of  Javan  man  are  not  to  be  found  in  any 
existing  apes  (the  structure  of  man  proves  this),  but  common 
ancestral  species  may  be  expected  far  back  in  Tertiary  times, 
although  such  have  not  yet  been  found.  Man  may  not  have 
been  exempt  from  evolution  (that  method  of  creation  which 
has  made  all  life  on  earth  akin),  and  inherited  apelike  char- 
acters of  teeth,  jaw,  skull,  and  other  parts  persisted  long  in 
early  species  of  mankind.  But  the  line  separating  the  human 
from  the  brute  was  passed  when  a  quickened  intelligence  was 
able  to  discern  the  relation  of  means  to  ends,  to  reason,  to 
conceive  abstract  ideas,  and  to  feel  moral  obligation.  In  the 
advent  of  man,  as  in  the  beginning  of  life  upon  the  planet, 
creative  energy  rose  to  a  higher  plane;  and  the  vast  possi- 
bilities of  self-directed  progress  now  opened  up  in  man  can 
well  be  compared  with  the  progress  which  life  had  already 
achieved  during  the  geologic  ages. 

Heidelberg  man.  The  next  glimpse  obtained  of  man's  long 
upward  path  is  after  a  lapse  of  scores  of  thousands  of  years, 


446  THE  ELEMENTS  OF  GEOLOGY 

wlien  another  race  emerges  in  western  Europe  during  the 
second  interglacial  epoch,  that  following  the  Kansan  glaciation. 
Of  this  forerunner  only  a  lower  jaw  has  been  discovered,  in  place 
beneath  seventy -nine  feet  of  loess  and  glacial  sands  near  Heidel- 
berg. This  jaw  is  the  largest  and  most  massive  of  all  known 
jaws  of  man.  It  bears  no  trace  of  a  chin  prominence  and  in 
various  other  characteristics  is  distinctly  apelike.  But  the  well- 
preserved  teeth  are  wholly  human.  Associated  with  the  jaw 

were  found  Eolithic  imple- 
^^^  ments  and  the  bones  of  an 

* . , :  f  J  extinct    elephant   and   rhi- 

noceros  and  the  cave  bear 
an(i  ]jon  —  animals  which 
then  roamed  widely  over 
southern  Europe.  Heidel- 
berg man  was  probably  the 
FIG.  373.  Jaw  of  Heidelberg  Man  ancestor  of  the  race  next 

mentioned. 

Neanderthal  man.  The  skeletal  remains  of  this  peculiar 
species  have  been  found  in  more  than  twenty  places  in  west- 
ern Europe.  Neanderthal  man  was  short,  averaging  about  five  feet 
four  inches  in  height,  with  broad,  stooping  shoulders,  a  short, 
thick  neck,  and  an  enormous  head  hung  forward  because  of  the 
curve  of  the  spinal  column.  The  magnum 'foramen,  the  open- 
ing admitting  the  spinal  cord  to  the  skull,  was  placed,  apelike, 
somewhat  in  the  rear  of  its  central  position  in  the  skull  of 
man  to-day.  The  knees  were  permanently  bent,  and  short  shin 
bones  show  a  plodding  gait.  As  with  Heidelberg  man,  his  jaw 
was  chinless.  As  with  Pithecanthropus,  overhanging  brow  ridges 
were  parted  from  a  low,  retreating  forehead  by  a  shallow  groove. 
During  the  Wisconsin  glaciation  the  Neanderthal ers  found 
shelter  in  the  caverns  and  under  the  overhanging  cliffs  of  the 
rivers  of  western  Europe.  In  these  cave  deposits,  and  some- 
times beneath  scores  of  feet  of  cave  earth,  river  sands,  and 


447 


448 


THE  ELEMENTS  OF  GEOLOGY 


stalagmitic  crusts,  are  found  not  only  their  fossil  skeletal 
remains  but  also  their  tools  and  weapons,  along  with  the 
bones  of  various  animals  on  which  they  fed.  Thus  we  know 
that  Neanderthal  man  was  a  hunting  savage.  He  had  learned 
the  art  of  kindling  fire.  He  flaked  flints  to  forms  suitable  for 
scrapers,  drills,  knives,  and  spearheads.  He  interred  his  dead 
with  ceremony.  A  Neanderthal  youth  of  about  sixteen  years, 
for  example,  was  found  in  a  cave  in  France  carefully  laid  to 

rest  upon  a  pillow  of  worked 
flints.  The  body  had  been 
extended  to  full  length,  the 
face  toward  the  rising  sun. 
The  head  rested  on  the  bent 
right  arm.  At  the  left  hand 
was  placed  a  masterpiece  of 
the  weapons  of  the  tribe. 
Charred  bones  of  joints  of 
beef  had  been  laid  about. 
These  primitive  men  felt 
pitying  love  and  sorrow  and  had  faith  in  life  beyond  the  grave. 
The  Neanderthal,  Heidelberg,  and  Java  races,  with  others' as 
primitive,  of  which  space  does  not  permit  description,  show 
that  the  human  stock  early  branched  into  different  species  and 
genera  of  sub-men,  which  long  since  vanished  from  the  earth. 
Cro-Magnon  man.  With  the  close  of  the  Wisconsin  glaciation 
the  slow,  squat,  stupid  Neanderthalers  were  replaced  by  a  race 
of  the  finest  physique  and  high  intelligence.  The  Cro-Magnons, 
as  these  immigrants  are  called,  were  one  of  the  tallest  races  of 
mankind.  Long  shin  bones  show  them  swift  of  foot.  The  pre- 
frontal  area  of  their  large  brain  was  well  developed,  and  face 
and  jaw  were  fully  human. 

The  Cro-Magnons  carried  well-nigh  to  perfection  the  art  of 
flaking  flint,  but  they  used  bone  more  largely.  Of  this  tough 
and  easily  worked  substance  they  made  needles,  which  imply 


FIG.  375.    Cro-Magnon  Drawing  of 
a  Reindeer 


THE  QUATERNARY 


449 


sewed  clothing,  tallies,  spoons,  whistles,  and  barbed  harpoons 
for  spearing  salmon.  Carved  wands,  used  perhaps  as  insignia 
of  office,  suggest  a  tribal  organization.  The  love  of  personal 
adornment  is  seen  in  armlets  and  necklaces  of  shells. 

To  the  Cro-Magnons  belongs  the  credit  of  the  world's  first 
art  in  drawing,  sculpture,  and  painting.  They  etched  on  plaques 
of  bone  and  ivory,  modeled  in  clay, 
sculptured  in  stone  or  bone,  and 
frescoed  on  the  walls  of  their  caves 
the  animals  about  them,  —  the  cave 
bear  and  lion,  the  mammoth  and 
woolly  rhinoceros,  the  wild  horse, 
the  reindeer,  boar,  and  bison.  The 
vigor,  freshness,  and  sure  touch  seen 
in  hundreds  of  their  works  of  art 
are  unapproached  among  untaught 
peoples.  But  as  their  art  galleries 
were  often  placed  in  remote,  dark 
chambers  of  their  caverns,  it  may 
be  supposed  that  these  frescoes  were 
somehow  connected  with  magic  rites 
employed  to  secure  success  in  hunt- 
ing. With  all  his  intelligence,  Cro- 
Magnon  man  was  still  a  hunting 
savage.  He  knew  no  agriculture, 


FIG.  376.  Head  of  Girl,  Cro- 
Magnon  Sculpture  in  Ivory 
(Enlarged) 

From  Brassempouy.  After  Piette, 
L'anthr.,  6,  PI.  6,  1895 


made  no  pottery,  wove  no  cloth,  and  domesticated  no  animals, 
—  not  even  man's  first  and  best  friend,  the  dog. 

The  Recent  epoch.  The  Glacial  epoch  ends  with  the  melting 
of  the  ice  sheets  of  North  America  and  Europe  and  the  replace- 
ment of  the  Pleistocene  mammalian  fauna  by  present  species. 
How  gradually  the  one  epoch  shades  into  the  other  is  seen  in 
the  fact  that  the  glaciers  which  still  linger  in  Norway  and 
Alaska  are  the  lineal  descendants  or  the  renewed  appearances 
of  the  ice  fields  of  Glacial  times. 


450          THE  ELEMENTS  OF  GEOLOGY 

Neolithic  man.  The  wild  Paleolithic  men  vanished  from 
Europe  with  the  wild  beasts  which  they  hunted,  and  their 
place  wTas  taken  by  tribes,  perhaps  from  Asia,  of  a  higher 
culture.  The  remains  of  Neolithic  man  are  found,  much  as 
are  those  of  the  North  American  Indians,  upon  or  near  the 
surface,  in  burial  mounds,  in  shell  heaps  (the  refuse  heaps  of 
their  settlements),  in  peat  bogs,  caves,  recent  flood-plain  deposits, 
and  in  the  beds  of  lakes  near  shore,  where  they  sometimes  built 
their  dwellings  upon  piles. 

The  successive  stages  in  European  culture  are  well  displayed 
in  the  peat  bogs  of  Denmark.  The  lowest  layers  contain  the  pol- 
ished stone  implements  of  Xeolithic  man,  along  with  remains  of 
the  Scotch  fir.  Above  are  oak  trunks  with  implements  of  bronze, 
while  the  higher  layers  hold  iron  weapons  and  the  remains  of  a 
beech  forest. 

Neolithic  man  in  Europe  had  learned  to  make  pottery,  to 
spin  and  weave  linen,  to  hew  timbers  and  build  boats,  and  to 
grow  wheat  and  barley.  He  had  domesticated  the  dog,  horse, 
sheep,  goat,  hog,  and  two  varieties  of  cattle  neither  of  which 
is  known  to  have  reached  Europe  before  this  time.  He  set  up 
huge  stones,  singly  or  in  lines  or  circles,  and  built  rude  tombs 
of  stone,  often  covered  with  great  mounds  of  earth.  Man  had 
not  yet  learned  the  art  of  extracting  metals  from  their  ores. 

In  North  America  no  proof  has  been  found  that  any  race 
ever  entered  the  continent  before  the  coining  of  the  immediate 
ancestors  of  the  American  Indian  from  Asia,  —  an  event  which 
seems  to  have  taken  place  after  the  close  of  the  Glacial  epoch. 

The  Neolithic  stage  of  culture  passed  into  the  age  of  bronze 
and  that  into  the  age  of  iron  at  different  times  with  different 
peoples.  Civilization  began  with  permanent  settlements,  as 
along  the  lower  Tigris  and  Euphrates  and  the  Nile.  And  with 
the  introduction  of  the  art  of  writing,  history,  based  on  written 
records,  takes  up  the  tale. 


THE  QUATERNARY  451 

Our  brief  study  of  the  outlines  of  geology  has  given  us,  it  is 
hoped,  some  great  and  lasting  good.  To  conceive  a  past  so  differ- 
ent from  the  present  has  stimulated  the  imagination,  and  to 
follow  the  inferences  by  which  the  conclusions  of  our  science 
have  been  reached  has  exercised  one  of  the  noblest  faculties  of 
the  mind,  —  the  reason.  We  have  learned  to  look  on  nature  in 
new  ways :  every  landscape,  every  pebble,  now  has  a  meaning 
and  tells  something  of  its  origin  and  history,  while  plants  and 
animals  have  a  closer  interest  since  we  have  traced  the  long 
lines  of  their  descent.  The  narrow  horizons  of  human  life  have 
been  broken  through,  and  we  have  caught  glimpses  of  that 
immeasurable  reach  of  time  in  which  nebulae  and  suns  and 
planets  run  their  courses.  Moreover,  we  have  learned  some- 
thing of  that  orderly  and  world-embracing  progress  by  which 
the  once  uninhabitable  globe  has  come  to  be  man's  well- 
appointed  home,  and  life  appearing  in  the  lowliest  forms  has 
steadily  developed  higher  and  still  higher  types.  Seeing  this 
process  enter  human  history  and  lift  our  race  continually  to 
loftier  levels,  we  find  reason  to  believe  that  the  onward,  up- 
ward movement  recorded  hi  the  geological  past  is  the  mani- 
festation of  the  same  wise  Power  which  makes  for  righteousness 
and  good,  and  that  this  unceasing  purpose  will  still  lead  011  to 
nobler  ends. 


INDEX 


Aa,  lava,  241 

Acadian  coal  field,  354 

Accretion  hypothesis,  304 

Acidic  rocks,  249 

Adelsberg  grotto,  47 

Adirondacks,  309,  816 

Africa,  357 

Agassiz,  Lake,  67,  111,  435 

Agates,  251 

Alabama,  317,  360 

Alaska,  85,  138,  140,  378 

Aletsch  glacier,  121 

Algae,  51,  52 

Algonlyan  group,  306,  310 

Allegheny  Mountains,  90,  224, 

403 

Alluvial  cones,  98 
Alluvium,  62 
Alps,   118,   121,   141,  210,  211, 

223,  229,  349,  427,  443 
Amazon  River,  175 
Ammonites,  294,  367,  380,  382 
Amphibians,  364,  383 
Amphicyon,  413 
Amygdules,  250 
Andes,  236,  279 
Angle  of  repose,  25 
Antarctic  continent,  294 
Antecedent  streams,  209 
Antelope,  413 
Anthracite,  281 
Anticlinal  folds,  203,  209 
Ants,  20 

Apennine  Mountains,  399 
Appalachia,  317,  351,  358 


Appalachian  coal  field,  356 

Appalachian  deformation,  358 

Appalachian  Mountains,   211,   214, 
•  218,  292 

Aquifer,  44 

Aragonite,  296 

Archseopteryx,  393 

Archeozoic  era,  306 

Arenaceous  rocks,  9 

Argillaceous  rocks,  9 

Arizona,  32,  75,  145,  151,  154,  220, 
229,  249,  257,  371,  390 

Arkansas,  337,  356,  373 

Arkose,  186,  282,  370 
326,       Artesian  wells,  44 

Arthropods,  322 

Artiodactyls,  411 

Assiniboine,  Mount,  34 
212,       Atlas  Mountains,  399 

Atmosphere,  304,  305 

Atolls,  191,  193 

Augite,  274 

Austin,  Tex.,  71 

Australia,  190,  357 

Avalanches,  26 

Bad  Lands,  397,  398 
Baltic  Sea,  170,  171,  199 
Barite,  287 

Barrier  Reefs,  191,  192 
Basal  conglomerate,  173,  184 
Basalt,  249 
Baselevel,  80,  83 
Basic  rocks,  249 
Basin  deposits,  103 
453 


454 


THE  ELEMENTS  OF  GEOLOGY 


Bay  bars,  164 

Beaches,  162,  164 

Bears,  413 

Bedding  planes,  5 

Belemnites,  382 

Belt  Mountains,  309 

Bergschrund,  121,  135,  137 

Bermudas,  148 

Birds,  392 

Bison,  413 

Bitter  Boot  Mountains,  272 

Black  Hills,  309,  371 

Blastoids,  339 

Blastosphere,  311 

Block  mountains,  222,  226 

Blowholes,  159 

Blue  Ridge,  309,  316 

Bomb,  volcanic,  256 

Bonneville,  Lake,  107,  198,  438 

Bosses,  270 

Bowlders,  erratic,  420 

of  weathering,  28 

Brachiopods,  323,  333,  343,  364,  380 
Brazil,  18,  236 
Breccia,  218,  255,  264 
British  Columbia,  373,  378 
Bronze  stage,  443,  448 
Bryozoans,  333 
Bunker  Hill,  422 

Calamites,  361,  367 
Calcareous  rocks,  9 
Calciferous  series,  327 
Calcite,  296 
Caldera,  239 

California,  24,  99,  136,  152,  158,  159, 
170,  197,  224,  256,  262,  287,  357, 
360,  371,  400 

Great  Valley  of,  101,  199,  372,  396 
Cambrian  period,  315 

glaciation  in,  358 

life  of,  319 


Camels,  412 

Canada,  28,  35,  67,  69,  90,  182,  198, 

200,  213,  218,  267,  307,  309,  316, 

336,  354,  357,  432,  437 
Cape  Breton  Island,  198 
Cape  Cod,  152 
Carbonated  springs,  261 
Carbonates,  "formation  of,  12 
Carboniferous  period,  350 

life  of,  361 
Carnivores,  413 
Cascade  Mountains,  90,  400 
Cats,  413 

Catskill  Mountains,  342 
Caucasus  Mountains,  399 
Caverns,  45,  241 
Cenozoic  era,  394 
Centipedes,  333 
Cephalopods,  324,  333,  339,  344,  367, 

380 

Ceratites,  380 
Ceratosaurus,  385 
Chain  coral,  339,  343 
Chalcopyrite,  287 
Chalk,  9,  374,  375 
Chalybeate  springs,  52 
Champlain  subsidence,  437 
Charleston  earthquake,  233 
Chazy  series,  327 
Chelan,  Lake,  141 
Chemung  series,  341,  342 
Chesapeake  Bay,  169,  170,  197 
Chicago,  146,  198,  436 
Chile,  235 
China,  28,  151 
Christmas  Island,  194,  248 
Cincinnati  anticline,  329,  356 
Cirques,  135 
Clinton  series,  335 
Coal,  352,  370,  375 
Coal  Measures,  351 
Coast  Range,  101,  372,  399 


INDEX 


455 


Coastal  plain,  Atlantic,  183 

Ccelenterates,  320 

Coke,  271 

Colorado,  18,  29,  33,  37,  153,  233, 

266,  271,  334 

Colorado  plateaus,  357,  403 
Colorado  River,  30,  75,  140, 154,  223, 

307,  313,  317 
Columbia  lavas,  400 
Columnar  structure,  253 
Concretions,  49 
Cones,  alluvial,  98 

volcanic,  257 
Conglomerate,  9,  173 
Congo  River,  175 
Conifers,  377 

Connecticut  valley,  370,  403 
Contemporaneous  lava  sheets,  248, 

268 

Continental  delta,  175,  183 
Continental  shelf,  183 
Continents,  188 
Contours,  69 
Copper,  287,  310 
Coquina,  177 
Coral  reefs,  188 

Corals,  ancient,  321,  332,  338,  379 
Cordaites,  363 
Cordilleran  ice  field,  425 
Corniferous  series,  341 
Coves,  161 
Crabs,  379 

Crandall  volcano,  263,  400 
Crater  Lake,  259 
Creodonts,  413 
Cretaceous  period,  372 
Crinoids,  332,  363,  379 
Crocodiles,  384 
Cro-Magnon  man,  448 
Cross  bedding,  65,  182 
Crustacea,  322,  332,  363,  379 
Crustal  movements,  195 


Cumberland  plateau,  90 

Cup  corals,  338 

Cycads,  377,  378 

Cycle  of  erosion,  84,  185,  292 

Cystoids,  321,  332,  367 

Dalmatia,  170 

Darwin's  theory  of  coral  reefs,  191 

Dead  Sea,  221,  279 

Death  Gulch,  264 

Deep-sea  deposits,  187 

Deer,  413 

Deflation,  152 

Deformation,  279 

Delaware  River,  197,  403 

Deltas,  108,  111,  197 

of  Ganges,  109 

of  Indus,  110 

of  Mississippi,  109,  197 
Denudation,  57 
Denver,  398 
Desert,  15,  55 
Devitrification,  257 
Devonian  period,  316,  341 
Dicotyls,  377,  404 
Digitigrade,  406 
Dikes,  244,  265 
Dinosaurs,  385 
Dinothere,  410 
Diorite,  274 
Dip,  202 
Dip  fault,  225 
Diplodocus,  286 
Dipnoans,  346 
Discina,  324 
Dismal  Swamp,  106 
Dogs,  413 
Dragon  flies,  364 
Drift,  18,  113,  416 

bowlders  of,  420 

englacial,  125 

extent  of,  426 


456 


THE  ELEMENTS  OF  GEOLOGY 


Drift,  pebbles  of,  114,  420 

stratified,  423 

thickness  of,  429 
Driftless  area,  438 
Drowned  valleys,  197 
Drumlins,  421 
Duluth,  436 
Dunes,  147 
Dust  falls,  145 

Earth,  age  of,  292,  298,  302 

interior  of,  276 
Earthquakes,  224,  233 

causes  of,  233,  237 

Charleston,  233 

distribution  of,  236 

geological  effects  of,  234 

India,  235 

Japan,  237 

New  Madrid,  236 
Earthworms,  20,  21 
Echinoderms,  321,  332,  333,  343, 

363 

Edentates,  441 
Egypt,  93 
Electric  Peak,  269 
Elephants,  410 
Elevation,  effects  of,  85 

'  movements  of,  197 
Eocene  epoch,  395 
Epicontinental  seas,  318 
Erratics,  133,  420 
Eskers,  424 
Etna,  248,  402 
Europe,   Pleistocene    ice    sheet    of, 

427 

Eurypterids,  333,  339,  363,  367 
Evolution,  300,  447 

Faceted  pebbles,  113,  114,  420 
Falls  of  the  Ohio,  343 
Fan  folds,  205 


Fault  scarps,  219 

Faults,  217 

Faunas,  299 

Feldspar,  9,  10,  42 

Ferns,  361 

Finger  lakes,  432 

Fire  clay,  353 

Fishes,  334,  339,  345,  364,  405 

Fissure  eruptions,  242 

Fissure  springs,  44 

Fjords,  139,  142 

Flint,  18,  375 

Flood  plains,  85,  93 

Floods,  54 

Floras,  299 

Florida,  46,  163,  177,  178,  188,  396 

Flow  lines,  252 

Fluorite,  287 

Folded  mountains,  210 

Folds,  201,  208 

Foliation,  283 

Foraminifera,  187,  374 

Forests,  Carboniferous,  354,  361 

Cretaceous,  377,  378 

Devonian,  343 

Tertiary,  404 
Fort  Wayne,  436 
Fossils,  177,  296 
Fractures,  215 
Fragmental  rocks,  8 
France,  167,  171 ;  cave  men  of,  445 
Fringing  reefs,  190 
Frogs,  383 
Frost,  15 

Funafuti  atoll,  194 
Fundy,  Bay  of,  182 

Gabbro,  274 
Ganges,  58,  109,  197 
Ganoids,  347 
Garnet,  281 
Gases,  volcanic,  244 


INDEX 


457 


Gastropods,  324 

Gastrula,  311 

Geneva,  Lake,  71 

Geodes,  49 

Geological  time,  divisions  of,  296 

Geology,  definition  of,  1,  3 

departments  of,  4 
Georgia,  18,  373 
Geysers,  52,  260 
Glacial  epoch,  142,  416 
Glaciers,  113 

abrasion  by,  133 

Alpine,  118 

compared  with  rivers,  137,  142 

crevasses  of,  123 

deposition  by,  138 

Greenland,  116 

lower  limit  of,  129 

melting  of,  126 

mode  of  formation,  118 

moraines,  124 

motion  of,  120,  122,  134 

piedmont,  131,  141 

plucking  by,  133 

tables,  130 

transportation  by,  132 

troughs,  137 

wells,  129 

young  and  mature,  129 
Glauconite,  176 
Globigerina  ooze,  187 
Giyptodon,  441 
Gneiss,  283 
Goats,  413 
Gold,  287,  372 
Goniatite,  344,  367 
Graded  slopes,  25 
Granite,  9,  274 
Graphite,  312 
Graptolites,  320,  339 
Gravitation,  22 
Great  Basin,  867,  360,  374,  376 


Great  Lakes,  198,  436 
Great  Plains,  82 
Great  Salt  Lake,  107 
Greenland,  115,  126,  378 
Green  Mountains,  309,  316,  420 
Green  sand,  176 
Ground  water,  39 
Ground  water  surface,  40 
Gryphsea,  379 
Gymnosperms,  363,  377 
Gypsum,  12,  335,  357,  371 

Hade,  217 

Hamilton  series,  341 

Hanging  valley,  139 

Hanging  wall,  217 

Hartz  Mountains,  214 

Hawaiian  volcanoes,  238,  248,  258, 

279 

Heat  and  cold,  13 
Heidelberg  man,  445 
Helderberg  series,  341 
Hematite,  310 
Henry  Mountains,  271,  375 
High  Plains,  100,  398 
Killers  Mountain,  271 
Himalayas,  122,  209,  210,  399 
^Historical  geology,  4,  291 
Honeycomb  corals,  339 
Hood,  Mount,  260,  262 
Hooks,  165 
Hornblende,  274 
Hornblende  schist,  284 
Hudson  Bay,  90,  170 
Hudson  River,  197,  493 
Hudson  series,  327,  329 
Humus  acids,  10 
Humus  layer,  19 
Huronian  systems,  308 
Hwang-ho  River,  151 
Hydrosphere,  22 
Hydrozoa,  320 


458 


THE  ELEMENTS  OF  GEOLOGY 


Icebergs,  116,  143 

Iceland,  242,  258 

Ichthyosaurus,  389 

Idaho,  34,  400  - 

Igneous  rocks,  9,  249,  250,  251,  278 

Illinoian  formation,  429 

Illinois,  54,  146,  356,  374 

India,  28,  102,  147,  235,  357,  402 

Indian  Territory,  356 

Indiana,  48,  104 

Indo-gangetic  plain,  101 

Indus  River,  101,  110 

Insects,  333,  364,  380 

Interior  of  earth,  276 

Internal  geological  agencies,  195 

Intrusive  masses,  270 

Intrusive  rocks,  273 

Intrusive  sheets,  268  * 

Inverness  earthquake,  236 

Iowa,  29,  69,  73,  80,  86,  336,  356, 

374,  431,  433,  439,  442 
lowan  formation,  429 
Iron  ores,  13,  53,  279,  310 
Islands,  coral,  188 
wave  cut,  159,  161 

Japan,  223,  224,  237 
Joints,  5,  31,  216 
Jordan  valley,  279 
Jura  Mountains,  141,  212 
Jurassic  period,  369 

Kame  terraces,  424 

Kames,  424 

Kansan  formation,  429 

Kansas,  41,  50,  100,  336,  357,  373, 

374,  429 
Kaolin,  12 
Karst,  47,  198 
Katahdin,  Mount,  420 
Keewatin  ice  field,  425 
Kentucky,  45,  46,  343,  442 


Keweenawan  system,  308,  310 
Kilauea,  239 

Kings  River  Canyon,  403 
Krakatoa,  246 

Labrador,  198 

Labrador  ice  field,  426 

Laccolith,  271 

Lagoon,  165,  167 

Lahontan,  Lake,  107,  438 

Lake  Chelan,  141 

Lake  dwellings,  448 

Lake  Geneva,'  71 

Lake  Superior  region,  284,  308,  310 

Lakes,  70,  222,  432 

basins,  97,  110,  127,  139,  141,  164, 
165,  167,  191,  221,  222,  235,  259, 
423,  432,  435 

deposits,  104 

glacial,  127,  139,  141,  423,  432,  435 

Pleistocene,  435 

salt,  106 
Laminae,  5 
Landslides,  26,  234 
Lapilli,  255 
Laramie  series,  375 
Laurentian  peneplain,  84,  308,  432 
Lava,  238,  241 
Lava  domes,  243,  400 
Lepidodenclron,  362,  367 
Lichens,  16 
Lignite,  271 

Limestone,  7,  177,  178,  190 
Limonite,  13 
Lingulella,  324 
Lithosphere,  21 
Lizards,  384 
Llamas,  412 
Loess,  150,  433 
Long  Island,  373 
Louisiana,  336,  396 
Lower  Silurian  period,  327 


INDEX 


459 


Luray  Cavern,  48 
Lycopods,  362 

Magnetite,  279,  310 

Maine,  169,  420 

Malaspina  glacier,  131 

Maldive  Archipelago,  193 

Mammals,  393,  405,  440 

Mammoth,  442 

Mammoth  Cave,  46 

Mammoth  Hot  Springs,  52 

Man,  414,  443 

Mantle  of  waste,  17 

Marble,  284,  329 

Marengo  Cavern,  48 

Marl,  104 

Marsupials,  393,  406 

Martha's  Vineyard,"  161,  373,  396 

Maryland,  56,  270 

Massachusetts,  106,   162,  267,   309, 

403,  417,  429 
Mastodon,  410,  441,  442 
Matterhorn,  34 
Maturity  of  land  forms,  80 
Mauna  Loa,  239 
Meanders,  96 
Medina  series,  335,  403 
Megatherium,  441 
Mendota,  Lake,  71 
Mesa,  31,  32,  153 
Mesozoic  era,  369 
Mesozoic  peneplain,  376,  403 
Metamorphism,  281 
Mexico,  373,  375 
Mica,  9 

Mica  schist,  284 
Michigan,  104,  356,  443 
Michigan,  Lake,  149,  198 
Mineral  veins,  49,  286 
Minnesota,  97,  426 
Miocene  series,  395 
Mississippi,  337 


Mississippi  embayment,  373, 374, 396 
Mississippi  River,  66,  67,  82,  94,  96, 

109 

Mississippian  series,  350 
Missouri,  18,  236 
Missouri  River,  56,  97 
Mobile  Bay,  197 
Mohawk  valley,  436,  437 
Molluscous  shell  deposits,  177 
Mollusks,  324 
Monadnock,  83 
Monkeys,  414 
Monoclinal  fold,  204 
Monocotyls,  377,  404 
Monotremes,  393,  406 
Montana,  71,  309,  313,  373 
Montreal,  268,  437 
Monuments,  33 
Moraines,  124 
Mosasaurs,  390 
Mountain  sheep,  413 
Mountains,  age  of,  229 

life  history  of,  212,  215 

origin  of,  90,  210,  222 

sculpture  of,  33,  137 
Movements  of  crust,  195 
Muir  glacier,  122,  129 

Nantucket,  373 

Naples,  201 

Narragansett  Bay,  197 

Natural  bridges,  46 

Natural  gas,  330 

Natural  levees,  93 

Nautilus,  334 

Neanderthal  man,  446 

Nebraska,  50,  82,  100,  255,  356 

Nebular  hypothesis,  304 

Neolithic  man,  444,  450 

Nevada,  1C4,  107,  222,  288,  289,  360, 

400 
Ne"ve",  120 


460 


THE  ELEMENTS  OF  GEOLOGY 


New  England,  88,  373,  376,  378,  395, 

403,  429,  432,  437 
Newfoundland,  198 
New  Jersey,  148,  166,  168,  176,  196, 

268,  269,  309,  310,  373,  437 
New  Madrid  earthquake,  236 
New  Mexico,  31,  371,  399 
New  York,  60,  90,  309,  327,  329,  335, 

336,  360,  421,  422,  423,  424,  432, 

443 

Niagara  Falls,  60,  199 
Niagara  series,  335 
Nile,  93,  109,  197 
Normal  fault,  217 
North  Carolina,  106 
North  Dakota,  67 
North  Sea,  170 
Notochord,  347 
Nova  Scotia,  198 
Nunatak,  116,  132 

Ohio,  82,  198,  329,  335,  441 

Ohio  Kiver,  55,  82 

Oil,  330 

Olenellus  zone,  323 

Olivine,  274 

Oolitic  limestone,  178 

Ooze,  deep-sea,  131 

Ordovician  period,  316,  327 

life  of,  331 

Oregon,  222,  262,  400 
Oreodon,  412 
Ores,  287,  290 
Organisms,  work  of,  16 
Oriskany  series,  341 
Ornithostoma,  392 
Orthoceras,  325,  367,  380 
Oscillations,  196 

a  cause  of,  273 

effect  on  drainage,  85 
Ostracoderms,  344 
Ottawa  River,  90 


Outcrop,  2 
Outliers,  31 
Outwash  plains,  425 
Oxidation,  13 
Oyster,  379,  380 

Pahoehoe  lava,  241 
Palseospondylus,  344 
Paleolithic  man,  444 
Paleozoic  era,  315 
Palisades  of  Hudson,  268 
Palms,  377 
Pamir,  15 
Peat,  94,  104 
Peccaries,  412 
Pelecypods,  324 
Pele"e,  Mt.,  246 
Peneplain,  83 

dissected,  86 

Laurentian,  89,  308,  402 

Mesozoic,  376,  403 
Pennsylvania,  211,  257,  357,  359,  403 
Pennsylvanian  series,  350,  351 
Perissodactyl,  408 
Perlitic  structure,  252 
Permian  series,  350,  357,  360,  366 
Petrifaction,  296 
Petroleum,  330,  343 
Phenacodus,  406 
Phyllite,  283 
Phyllopod,  323 

Piedmont  Belt,  87,  214,  309,  374 
Piedmont  plains,  99 
Pikes  Peak,  18 

Pithecanthropus  erectus,  444-5 
Placers,  287 

Plains  of  marine  abrasion,  172 
Planation,  81 

Planetesiinal  hypothesis,  304 
Plantigrade,  406 
Platte  River,  82 
Playa,  103 


INDEX 


461 


Playa  lakes,  104 
Pleistocene  epoch,  416 
Plesiosaurus,  389,  390 
Pliocene  epoch,  395 
Plucking,  133 
Po  River,  68,  197 
Pocono  sandstone,  360,  404 
Porosity  of  rocks,  40 
Porphyritic  structure,  262 
Potholes,  59 

Potomac  River,  68,  66,  403 
Predentata,  386 
Primates,  414 

Prince  Edward  Island,  198 
Proboscidians,  410,  441,  442 
Proterozoic  era,  306 
Pteropods,  325 
Pterosaurs,  391 
Puget  Sound,  396 
Pumice,  260 
Pyrite,  13 

Quarry  water,  15 
Quartz,  6,  9 
Quartz  schist,  284 
Quaternary  period,  395,  416 
Quebec,  28 

Rain,  erosion,  23 

Rain  prints,  181 

Recent  epoch,  416,  440,  449 

Re  concentration  of  ores,  289 

Record,  the  geological,  291 

Red  clay,  187 

Red  River  of  the  l?orth,  67 

Red  Sea,  221 

Red  snow,  115 

Reefs,  coral,  188 

Regional  intrusions,  272 

Reptiles,  367,  383 

Rhinoceros,  408 

Rhizocarp,  343 

Rhode  Island,  356 


Rhone  glacier,  123 
Rhyolite,  249 
Richmond,  Va.,  370 
Rift  valleys,  221 
Ripple  marks,  180 
Rivers,  64 

bars,  66 

braided  channels,  94 

deltas,  108 

deposition,  62 

discharge,  66 

erosion,  69 

estuaries,  86 

flood  plains,  93 

floods,  64 

graded,  74 

gradients,  82 

load  of,  66 

mature,  72,  80,  97,  98 

meanders,  96 

plains,  99 

profile  of,  73 

revived,  86 

run-off,  54 

structure  of  deposits,  102 

terraces,  96 

transportation,  66,  64 

waterfalls,  78 

young,  67 

Roches  moutonne'es,  134,  418 
Rock  bench,  156 
Rock  salt,  12,  357,  371 
Rocky  Mountains,  376,  399,  437 
Ruminants,  412 

Saber-tooth  tiger,  413 
Saguenay  River,  90,  201 
Sahara,  15,  146,  150 
St.  Elias  Range,  399 
St.  Peter  sandstone,  160 
Salamanders,  383 
Salina  series,  335 


462 


THE  ELEMENTS  OF  GEOLOGY 


Salt,  common,  106,  335 
Salt  lakes,  106 
San  Francisco  Bay,  197 
Sand,  beach,  163 

of  deserts,  149 

reefs,  165,  167 

storms,  145 
Sandstone,  6,  7,  186 
Sarcoui,  258 
Sauropoda,  386 
Schist,  283 
Schladebach,  277 
Scoria,  250,  255 
Scorpions,  339,  340,  363 
Scotland,  170,  220,  402 
Sea,  155 

erosion,  156 

deposition,  174 

transportation,  162 
Sea  arch,  159 
Sea  cave,  158 
Sea  cliff,  156,  157 
Sea  cucumber,  363 
Seals,  414 
Sea  stacks,  169 
Sea  urchin,  332,  379 
Seaweed,  176 
Sedimentary  rocks,  8,  9 
Selkirk  Mountains,  218 
Septa,  338 
Sequoia,  378 
Shale,  8,  9 
Sharks,  345,  405 
Shasta,  Mount,  262,  400 
Sheep,  413 

Shenandoah  valley,  403 
Shores  of  elevation,  167 
Shores  of  depression,  169 
Siderite,  53 

Sierra  Nevada  Mountains,  24,  90,  99, 
224,  229,  272,  287,  318,  357,  371, 
372,  396,  398,  399,  402,  437 


Sigillaria,  362,  367 

Silica,  6,  178 

Silurian  period,  316,  334 

life  of,  338 
Sink  hole,  46 
Slate,  207,  282 
Slaty  cleavage,  207 
Slickensides,  217 
Snake  River  lavas,  400,  401 
Snakes,  384,  405 
Soil,  19 
Solfatara,  260 
Solution,  11 
Soufriere,  246 
South  America,  357 
South  Carolina,  233 
South  Dakota,  276,  374,  397 
Spanish  Peaks,  271,  376 
Spherulites,  252 
Spiders,  363 
Spitzbergen,  378 
Sponges,  320,  379 
Springs,  41 

thermal,  50 
Stalactite,  48 
Stalagmite,  48 
Starfishes,  332 
Staubbach,  140 
Stegosauras,  387 
Stoss  side,  134 
Stratification,  5,  64,  180 
Strise,  glacial,  114,  133,  418 
Strike,  203 
Strike  fault,  225 
Stromatopora,  331,  379 
Stromboli,  244 
Subsidence,  85,  183,  197 
Sun  cracks,  180 
Superior,  Lake,  257 
Superposition,  law  of,  293 
Susquehanna  River,  403 
Sutlej  River,  209 


INDEX 


463 


Sweden,  199 
Swine,  412 
Switzerland,  28,  427 
Syenite,  274 
Synclinal  fold,  204 
Syracuse,  N.Y.,  436 
Syringopora,  339 

Tabulae,  339 

Taconic  deformation,  329 

Taconic  Mountains,  376 

Talc,  284 

Talc  schist,  284 

Talus,  23 

Tapir,  409 

Teleost  fishes,  349,  382,  405 

Tennessee,  90,  373 

Terminal  moraines,  126,  422,  432 

Terraces,  86,  96 

Tertiary  period,  395 

Texas,  15,  69,  71,  166,  336,  356,  357, 

371,  373,  374,  378 
Theromorphs,  383 
Throw,  217 
Thrust  faults,  217 
Till,  418  ;  till  plains,  420 
Toronto,  443 
Trachyte,  249,  258 
Travertine,  52 
Trenton  series,  327 
Triassic  period,  369 
Triceratops,  387 

Trilobites,  322,  332,  339,  363,  367 
Tuff,  255 
Turkestan,  103 
Turtles,  384 
Tyrannosaur,  386 

Unconformity,  227 

Undertow,  174 

Utah,  107,  271,  360,  371,  398,  399 

Utica  series,  327 


V-Valleys,  74 

Valley  drift,  128 

Valley  trains,  425 

Valleys,  66 

Vermont,  309,  329,  420 

Vernagt  glacier,  129 

Vertebrates,  334,  349 

Vesuvius,  247,  259,  402 

Virginia,  48,  84,  106,  370,  403,  442 

Volcanic  ashes,  244,  255 

cones,  257 

necks,  267 

rocks,  249 
Volcanoes,  238 

causes  of,  278 

decadent,  260 

submarine,  248 

tertiary,  399 

Walrus,  414      . 
Warped  valleys,  101 
Warping,  198 
Wasatch  Mountains,  375 
Washington,  18,  91,  150,  400 
Waterfalls,  59,  78 
Waves,  156 
Weathering,  5 

chemical,  10 

differential,  29 

mechanical,  13 
Wells,  41 

artesian,  44 

West  Virginia,  79,  357,  359 
White  Mountains,  443 
Wind,  144 

deposition,  147 

erosion,  151 

pebbles  carved  by,  152 

transportation,  145 
Wisconsin,  15,  18,  70,  71,  90,  94,  422, 

426 

Wisconsin  formation,  429,  431 
Wyoming,  50,  98,  371 


464  INDEX 

Yahtse  River,  131  Zeuglodon,  414 

Yellow  Sea,  151,  170  Zone  of  cementation,  49,  180 

Yellowstone  canyon,  74  Zone  of  solution,  45 

Yellowstone  National  Park,  50,  51,       Zones  of  flow  and  of  fracture,  207 

52,  260,  261,  263,  269,  400 
Yosemite,  403 


ANNOUNCEMENTS 


THE  ELEMENTS  OF  GEOLOGY 

By  WILLIAM  HARMON  NORTON 

Alumni  Professor  of  Geology  in  Cornell  College,  Mt.  Vernon,  Iowa 


8vo,   cloth,   461    pages,   illustrated 


THE  essentials  of  the  science  of  geology  are  treated 
with  fullness  and  ample  illustration  in  this  text-book 
for  beginners.  By  limiting  his  discussion  chiefly  to  this 
continent  the  author  has  been  able  to  devote  a  large 
amount  of  space  to  the  principles  which  he  describes.  The 
following  characteristics  are  important. 

1.  The  outline  is  exceptionally  simple.    Under  the  leading  geological 
processes  are  grouped  the  rock  structures  and  land  forms  of  which 
they  are  the  cause. 

2.  The    inductive    method    is    emphasized    throughout.      Concrete 
examples  are  given  large  space  as  the  basis  of  generalizations  of  the 
science.     Numerous  exercises  and  problems,  many  of  which  are  in  the 
form  of  diagrams,  are  designed   to   train  the  pupil  and  to  test  his 
knowledge. 

3.  The  cycle  idea  is  made  prominent,  and  both  the  records  of  erosion 
and  those  of  sedimentation  are  given  special  attention. 

4.  In  historical  geology  a  broad  view  is  afforded  of  the  development 
of  the  North  American  continent  and  of  the  evolution  of  life  upon  the 
earth.     Only  the  leading  types  of  plants  and  animals  are  mentioned, 
and  special  attention  is  given  to  those  which  mark  the  lines  of  descent 
of  forms  now  living. 

The  book  is  designed  for  use  in  high  schools  and  acad- 
emies, and  may  also  be  found  useful  in  short  elementary 
college  courses. 


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